Stages in Chick Embryo Development

The development of a chick embryo from a freshly laid hen’s egg involves various stages and intricate processes. Let’s explore the initial stages of chick embryo development based on the provided information:

The hen’s egg, when laid, is relatively large, measuring about 3cm in diameter and 5cm in length. It is considered a macrolecithal egg due to its substantial amount of yolk. The egg has an oval shape, and within it, the ovum can be observed. The ovum contains a nucleus and is surrounded by yolk-free cytoplasm. The ovum has a diameter of approximately 3mm and is located at the animal pole of the egg.

The yolk present in the egg fills the entire space and consists of alternating layers of yellow and white. These layers are arranged concentrically around a flask-shaped structure called the latebra. Below the blastodisc (the embryonic disk), the neck of the latebra expands, forming the nucleus of Pander. The yellow yolk obtains its color from carotenoids, while the white yolk layers are thinner compared to the thick yellow yolk layers. The yolk itself is in a liquid state and contains around 49% water, 33% phospholipids, and 18% proteins, vitamins, and carbohydrates.

Covering the entire ovum is the plasma membrane, also known as the plasmalemma. This membrane is composed of lipoproteins and provides protection to the ovum. Additionally, the ovum is enveloped by egg membranes, which further safeguard the developing embryo.

These initial stages lay the foundation for the subsequent development of the chick embryo. As the embryo grows, more intricate processes and stages, such as gastrulation, neurulation, and organogenesis, occur, leading to the formation of various organs and systems. It’s important to note that the complete development of the chick embryo from these initial stages to hatching is a fascinating and complex process involving precise genetic and environmental interactions.

For a comprehensive understanding of chick embryo development, it is recommended to consult additional resources or seek the guidance of experts in the field.

The development of a chick embryo inside an egg is a fascinating process that involves intricate stages and transformations. The fully formed hen’s egg, upon being laid, possesses distinctive characteristics that contribute to the development of the embryo within. Understanding the various stages of chick embryo development provides valuable insights into the complex biological processes at play.

Macroscopic Features of the Hen’s Egg: A freshly laid hen’s egg is relatively large, measuring 3cm in diameter and 5cm in length. It is classified as a macrolecithal egg due to its substantial yolk content. The egg is oval in shape, and the ovum, situated on the animal pole, contains a nucleus enveloped by yolk-free cytoplasm. With a diameter of approximately 3mm, the ovum’s cytoplasm is visible.

Yolk Composition and Structure: The egg is filled with yolk, which consists of alternating layers of yellow and white concentric layers. These layers surround a flask-shaped structure known as the latebra. Below the blastodisc, the neck of the latebra expands, forming the nucleus of Pander. The yellow color of the yolk is attributed to carotenoids, while the white layers are thinner in comparison. Yolk primarily comprises 49% water, 33% phospholipids, and 18% proteins, vitamins, and carbohydrates.

Egg Membranes: The entire ovum is enclosed by the plasma membrane, also referred to as the plasmalemma. This lipoprotein layer protects the ovum and is further surrounded by egg membranes. The primary membranes, namely the vitelline membrane, are secreted by follicle cells and originate from two sources: the ovary and the fallopian tube. The secondary membranes are produced by the oviduct and include the albumen, a white substance containing water and proteins. The albumen consists of three layers: thin albumen, thick albumen (or dense albumen), and the innermost chalazae, which act as balancers to maintain the ovum’s central position.

Shell Structure: Two shell membranes are located above the albumen, and an air space forms between these membranes towards the broad end of the egg. The creation of this air space occurs when the egg cools after being laid, reducing the temperature from 60°C. The outermost layer is the porous calcareous shell, which facilitates gas exchange. Initially, the freshly laid hen’s egg has a soft shell that gradually hardens over time.

Laying of the Egg and Incubation: The hen typically expels the egg from the cloaca between 9 A.M. and 3 P.M. At the time of laying, the formation of the endoderm, one of the primary germ layers, is complete. To allow further development, the egg needs to be incubated. Incubation involves maintaining the egg at a constant temperature of 38°C, which is achieved when the hen sits on the egg. Alternatively, eggs can be incubated artificially in incubators. The hatching process generally requires 21 days.

Fertilization and Early Development: Fertilization occurs in the upper region of the oviduct. One sperm penetrates the hen’s egg and fertilizes it, initiating the development of the embryo. The fertilized egg then traverses through the oviduct, a journey that takes approximately 22 hours. Consequently, the early stages of embryonic development occur within the oviduct.

Understanding the stages in chick embryo development is crucial for comprehending the intricate processes that occur within the egg. From the initial formation of primary and secondary membranes to the development of the shell and the subsequent incubation period, each stage contributes to the growth and maturation of the chick embryo until it is ready to hatch.

Cleavage

Cleavage is an essential process in the development of a chick embryo. In this case, cleavage is restricted to the blastodisc, while the yolk remains uncleaved, resulting in a meroblastic or discoidal cleavage pattern. The blastodisc consists of a central whitish and circular region surrounded by a darker marginal zone called the periblast, which merges with the underlying white yolk.

The process of cleavage can be divided into several stages:

1. First Cleavage: Approximately five hours after fertilization, the first cleavage occurs. It is meridional, meaning it is restricted to the center of the blastodisc. However, the cleavage is incomplete, and distinct blastomeres (individual cells resulting from cleavage) are not formed.
2. Second Cleavage: This cleavage occurs at right angles to the first cleavage. However, clear blastomeres are still not formed as a result of this division.
3. Third Cleavage: This cleavage is vertical and parallel to the first division, occurring on the two sides of the first division. As a result, eight blastomeres are formed, but they do not exhibit clear boundaries.
4. Fourth Cleavage: During this cleavage, eight central blastomeres and eight peripheral blastomeres, known as marginal blastomeres, are formed. It is at this stage that definite cells are observed. The central eight cells become completely separated from the yolk. After the fourth cleavage, subsequent cleavages become irregular, leading to the formation of a blastoderm.

Throughout these cleavages, the furrows do not extend to the edge of the disc. As a result, blastomeres in the central area have distinct boundaries, while those in the outer area merge with the unsegmented syncytial periblast.

The syncytial periblast, which is found at the periphery of the blastoderm, facilitates nutritive contact between the yolk and the growing mass of cells in the blastoderm. The central cellular area expands as cells from the periphery are added. Eventually, horizontal divisions occur, and the central area becomes two or three cells thick. It becomes separated from the underlying yolk by a space known as the subgerminal cavity or blastocoel. This cavity forms either through splitting or the separation of the upper layer from the lower layer that maintains a connection with the yolk mass.

Blastula

The blastula stage is a crucial phase in the development of a chick embryo. During this stage, the cells of the blastoderm undergo rapid division, leading to the formation of a mass of cells above the segmentation cavity in several layers. These cells have distinct boundaries and are present in the central region of the blastoderm.

The blastoderm can be divided into two distinct areas:

1. Area Pellucida: The central part of the blastoderm, known as the area pellucida, consists of four to five layers of cells that are lifted from the yolk. This region is free from yolk and appears transparent. It is destined to become the embryo proper.
2. Area Opaca: The region at the zone of junction where the cells are in contact with the yolk is called the area opaca. This area gives rise to extra-embryonic structures. Over time, the area opaca becomes differentiated into three distinct zones. The original periblast blastomeres form a germ wall, and the addition of more blastomeres on the periphery leads to the formation of an outer ring called the margin of overgrowth. The blastomeres in this zone do not have well-defined boundaries. Inner to the germ wall area, there is a group of cells in close contact with the yolk that lacks complete cell boundaries. This region is known as the zone of junction.

During this stage, the blastoderm consists of two types of cells: relatively large yolk-laden blastomeres and smaller yolk-free blastomeres. These blastomeres undergo segregation, with the yolk-rich blastomeres gradually accumulating at the underside of the blastoderm, while the smaller yolk-poor blastomeres remain at the surface. The upper layer is called the epiblast, while the lower layer is known as the hypoblast. A narrow cleft called the blastocoel appears between the epiblast and hypoblast. The separation of the epiblast from the hypoblast is called delamination.

The result of cleavage is the transformation of the blastodisc into a disc-shaped blastula known as a discoblastula. The blastula floats on top of the yolk mass, and this stage marks an important milestone in the development of the chick embryo.

Gastrulation In Chick

• Gastrulation in the chick embryo is a complex process that occurs within a relatively short timeframe. It can be divided into two main phases: endoderm formation and primitive streak formation, along with the movement of chordamesodermal elements. The entire process takes approximately 22 hours to complete, beginning around four to five hours after the onset of incubation.
• During the formation of the endoderm, the hypoblast layer develops as a single layer of cells on the inner side of the blastocoel. Once the endoderm forms, the upper layer is referred to as the epiblast. There are several theories explaining how the endoderm is formed.
• The infiltration theory, proposed by Peter in 1923, suggests that some yolk-laden cells in the blastoderm fall into the blastocoel. These cells migrate forward from the posterior end of the blastoderm, one behind the other, ultimately giving rise to the endoderm.
• The delamination theory, proposed by Spratt in 1946, suggests that the blastoderm is initially two or three layers thick. The lower layer, known as the endoderm, separates from the upper layers (ectoderm) through a splitting process. The blastocoel is situated between the ectoderm and endoderm.
• According to the theory of involution proposed by Peterson in 1909, a slit-like opening forms at the posterior side of the blastoderm. Through this opening, blastoderm cells roll into the primary blastocoel, forming the endoderm.
• The theory of invagination, proposed by Jockobson in 1938, states that the posterior end of the blastoderm invaginates into the blastocoel, forming a small pocket that becomes the endoderm.
• The second step in gastrulation involves the formation of the primitive streak. It appears as a thickened area at the posterior region of the area pellucida, along the mid-dorsal line. The primitive streak starts to develop approximately eight hours after incubation. Initially, it is short and broad but gradually extends toward the middle of the blastoderm. By 18 to 19 hours of incubation, the definite primitive streak is formed. Within the primitive streak, a narrow furrow called the primitive groove develops, with thick edges known as primitive folds. At the anterior end of the groove, there is a mass of closely packed cells called Hensen’s node or primitive knob, which contains a pit known as the primitive pit, representing the vestige of the neurenteric canal.
• As the primitive streak elongates, the area pellucida also elongates. Cells from the region of the primitive streak migrate inward, a process called immigration, and become the prechordal plate, notochord, and mesoderm.
• The mesoderm is formed as two layers. In front of the primitive streak, an area without mesoderm called the proamnion is present, where the head develops. After approximately 48 hours of incubation, the proamnion is also occupied by mesoderm. The mesoderm is further divided into dorsal, intermediate, and lateral mesoderms.
• The notochordal cells arrange themselves to form a cylindrical rod called the notochordal process, which grows from Hensen’s node. As the notochordal process grows, the primitive streak gradually diminishes and incorporates into the tail bud.
• The dorsal mesoderm is located on either side of the notochord and is divided into segments called somites. The first pair of somites forms after approximately 21 hours of incubation, with subsequent pairs forming at a rate of one pair per hour. By 24 hours of incubation, the embryo contains four pairs of somites.
• The intermediate mesoderm connects the dorsal mesoderm with the lateral mesoderm as a stalk. It later undergoes segmentation and gives rise to the kidneys.
• The lateral mesoderm extends along the periphery of the embryo and can be divided into extraembryonic and embryonic mesoderms. The lateral mesoderm splits into two layers, with the upper layer known as the somatic mesoderm and the inner layer called the splanchnic mesoderm. The combination of ectoderm and somatic mesoderm is referred to as the somatopleure, while the splanchnic layer and endoderm are known as the splanchnopleure. The space between the two layers of mesoderm is called the coelom.
• By the end of gastrulation, specific organ-forming areas have started to develop, setting the stage for further embryonic development in the chick embryo.

24 Hrs. Chick Embryo

• The 24-hour chick embryo undergoes significant developmental changes, marking an important stage in its growth and morphogenesis. At this point, several key features and structures have emerged, shaping the embryo’s future form and function.
• Firstly, the 24-hour chick embryo takes on an oval shape, indicating its growth and differentiation during the incubation period. This shape represents the overall organization of the embryo at this stage.
• The primitive streak, a crucial structure in embryonic development, is fully formed by this time. The process of gastrulation, which involves the rearrangement of cells to form the three germ layers (ectoderm, mesoderm, and endoderm), is also completed. Gastrulation is a critical stage in which the body plan begins to take shape.
• One notable feature at this stage is the extension of the notochord from the Hensen’s node as a head process into the mesoderm-free area anteriorly. The notochord plays a vital role in the development of the vertebrate embryo, providing structural support and signaling molecules for proper development.
• The head fold and fore-gut also begin to develop during this period. These structures are essential for the formation of the head and the initial development of the digestive system.
• The mesoderm, one of the three germ layers, differentiates into distinct regions: somites, intermediate plate mesoderm, and lateral plate mesoderm. In the 24-hour chick embryo, four pairs of somites have already differentiated from the mesoderm. Somites serve as the basis for the development of various structures such as muscles, vertebrae, and dermis.
• Additionally, the coelom, a fluid-filled cavity that will later become body cavities, starts to develop in the lateral plate mesoderm. The coelom provides space for the organs to develop and also plays a role in their movement and function.
• In terms of circulatory development, blood islands appear in the area opaca, which is a region of the developing embryo. The pericardial region, which will house the heart, is also established. These early indications of circulatory development are crucial for the embryo’s overall growth and survival.
• Further modifications occur in the area opaca, transforming it into the area vasculosa and area vitellina. These changes contribute to the vascularization and nourishment of the embryo.
• Regarding the nervous system, the neurectoderm gives rise to the neural folds and neural groove. At this stage, the fusion of the neural folds begins from the mid-region. This fusion will ultimately form the neural tube, which will develop into the brain and spinal cord.

In summary, the 24-hour chick embryo exhibits numerous milestones in its development. From the formation of the primitive streak and completion of gastrulation to the differentiation of mesoderm into somites, the embryo is rapidly progressing toward the formation of vital structures and organs. The emergence of blood islands, the establishment of the pericardial region, and the modifications in the area opaca signify the early stages of circulatory development. Additionally, the neurectoderm’s development into neural folds and the initiation of neural fold fusion highlight the formation of the future nervous system. These intricate processes and structures set the stage for further growth and development in the chick embryo.

33 Hrs Chick Embryo

• The 33-hour chick embryo represents a crucial stage in its development, characterized by significant changes and the emergence of key structures. These developmental milestones contribute to the formation and differentiation of various systems within the embryo.
• One notable development during this period is the lengthening of the foregut and the subcephalic pocket. These structures are essential components of the developing digestive system, providing the foundation for future gastrointestinal structures and functions.
• The neural tube, a fundamental structure in nervous system development, is formed at this stage. Simultaneously, the sinus rhomboidalis, a cavity within the neural tube, also begins to take shape. The neural tube gives rise to the brain and spinal cord, while the sinus rhomboidalis is involved in the development of the hindbrain.
• The encephalon, or the developing brain, undergoes primary division into three regions: the prosencephalon, mesencephalon, and rhombencephalon. This division sets the groundwork for the subsequent specialization and differentiation of different brain regions, each responsible for specific functions.
• On either side of the neural tube, neural crest cells begin to form. Neural crest cells are a unique group of cells that migrate to various locations in the embryo, where they contribute to the development of diverse structures, including parts of the peripheral nervous system, craniofacial skeleton, and pigment cells.
• Another significant development is the formation of the infundibulum, which is a median ventral outgrowth from the floor of the prosencephalon. The infundibulum plays a crucial role in the regulation of hormone production and the connection between the nervous and endocrine systems.
• During this stage, the embryo also develops 13 pairs of somites. Somites are segments of mesoderm that give rise to various structures, such as muscles, vertebrae, and dermis. The differentiation of somites marks the formation of the embryo’s segmented body plan.
• The heart begins to take shape as a tubular structure located in the midventral region to the foregut. This early development of the heart is a critical step in the formation of the circulatory system and the embryo’s ability to supply nutrients and oxygen to its growing tissues.
• Both extraembryonic and intraembryonic blood vessels begin to form. The area vasculosa, which previously underwent modifications, now develops extraembryonic blood vessels. These vessels are responsible for the exchange of nutrients and waste between the embryo and the surrounding environment. Simultaneously, the embryo itself begins to develop intraembryonic blood vessels, which will become integral to the circulatory system.
• Lastly, the primitive streak, a prominent feature in earlier stages of development, disappears during the 33-hour period. The disappearance of the primitive streak signifies the completion of its role in the gastrulation process and marks a transition to the next stage of embryonic development.

In summary, the 33-hour chick embryo demonstrates significant advancements in its development. The lengthening of the foregut and subcephalic pocket, the formation of the neural tube, and the primary division of the encephalon into prosencephalon, mesencephalon, and rhombencephalon shape the embryo’s neurological structures. The emergence of somites and the formation of the heart and blood vessels lay the foundation for the musculoskeletal and circulatory systems. Finally, the disappearance of the primitive streak marks a significant milestone in the embryo’s developmental progression.

48 Hrs. Chick Embryo

• The 48-hour chick embryo represents a critical stage in its development, marked by various structural and functional advancements. During this period, several key processes and formations occur, shaping the embryo’s overall form and function.
• One notable development is the appearance of cranial flexure and torsion. These changes involve the bending and twisting of the head region, which is essential for proper alignment and positioning of the developing brain and sensory structures.
• The brain undergoes further differentiation during this stage, with the formation of eleven neuromeres. The prosencephalon, or forebrain, consists of three neuromeres, while the mesencephalon, or midbrain, consists of two neuromeres. The rhombencephalon, or hindbrain, consists of six neuromeres. These neuromeres serve as organizing centers for the future development of specific brain regions and functions.
• Secondary constrictions begin to form in the brain at this stage. These constrictions play a role in further subdividing and organizing the developing brain into distinct regions.
• The vitelline circulatory system, which refers to the extraembryonic circulation involving the yolk sac, is fully established by the 48-hour mark. This system provides essential nutrients and oxygen to the growing embryo during this early stage of development.
• Simultaneously, the development of intraembryonic blood vessels continues. These vessels contribute to the formation of the embryonic circulatory system, which will eventually connect to the developing heart.
• Formation of two pairs of aortic arches also occurs during this period. The aortic arches represent early blood vessel structures that will eventually form important components of the circulatory system, supplying blood to different regions of the embryo.
• The heart undergoes significant changes, including twisting and the formation of chambers. This twisting is essential for proper alignment of the heart within the developing body and ensures efficient blood flow. The formation of chambers within the heart sets the stage for the establishment of a functional four-chambered heart.
• With the commencement of blood circulation, the embryo’s circulatory system becomes functional, allowing for the distribution of oxygen and nutrients throughout the developing body.
• The formation of the optic cup and lens vesicle takes place during this period, representing crucial developments in the embryonic visual system. These structures lay the foundation for the future formation of the eyes.
• A visceral cleft, or pharyngeal cleft, begins to form. These clefts are openings in the developing neck region and are involved in the formation of structures such as the jaw and throat.
• The auditory vesicle, an early precursor to the inner ear, also develops during this stage. This structure is vital for the embryo’s future ability to perceive sound and maintain balance.
• Additionally, the 48-hour chick embryo differentiates into 28 somites. Somites are segmented structures that give rise to various tissues, including muscles, vertebrae, and dermis. The differentiation of somites further contributes to the embryo’s overall body plan.
• The development of extraembryonic membranes progresses, aiding in the embryo’s protection, nourishment, and gas exchange. These membranes include the amnion, chorion, and allantois, among others.
• Lastly, the pronephros, the most anterior of the three successive pairs of embryonic kidneys, begins to form during this stage. The pronephros represents an early developmental stage of the excretory system.

Foetal Membranes Of Chick

The foetal membranes of the chick play a crucial role in the development and protection of the embryo. These membranes are derived from specific regions of the blastoderm, which is the structure formed during the early stages of embryonic development.

In the chick, the extra-embryonic membranes are well developed and serve various functions to support and nourish the growing embryo. These membranes are formed from the marginal blastoderm, which surrounds the central part that will give rise to the embryo proper.

The two main extra-embryonic membranes in the chick are the amnion and chorion, both of which develop from the somatopleure. The somatopleure is a layer of cells that will form the body wall of the embryo. The amnion is a fluid-filled sac that surrounds and protects the developing embryo, providing a cushioning environment. The chorion, on the other hand, surrounds the amnion and plays a vital role in gas exchange between the embryo and the external environment.

Another set of foetal membranes, the yolk sac and allantois, develop from the splanchnopleure. The splanchnopleure is a layer of cells that will form the lining of the gut and other internal organs. The yolk sac functions as a site of nutrient storage for the embryo, providing essential nourishment during its development. The allantois, on the other hand, is involved in waste storage and gas exchange, helping to remove metabolic waste products and facilitate respiration.

Together, these foetal membranes of the chick contribute to the care, maintenance, and support of the developing embryo. The amnion and chorion provide protection and regulate gas exchange, while the yolk sac and allantois play crucial roles in nutrient storage, waste removal, and gas exchange. These extra-embryonic membranes ensure the proper development and survival of the chick embryo by providing a supportive and nourishing environment throughout its growth.

Amnion & Chorin

• The amnion and chorion are closely associated structures involved in the development and protection of the embryo. The amnion acts as a bag-like covering that separates the embryo from the internal environment. It is formed from somatopleuric amniotic folds, which include the head fold, lateral folds, and tail fold.
• The development of the amniotic folds follows a specific sequence. At around 30 hours of incubation, the amniotic head fold forms in front of the embryo’s head. Around the third day of incubation, the amniotic tail fold grows in the opposite direction. Meanwhile, the lateral folds develop and grow dorso-medially. Eventually, the head fold, lateral folds, and tail fold fuse near the posterior end of the embryo. Initially, they may show an opening called the amniotic umbilicus before eventually uniting.
• The amniotic folds consist of two limbs: an outer limb and an inner limb. Both limbs consist of ectoderm and a thin layer of somatic mesoderm. In the outer limb, the ectoderm is external, while the mesoderm is internal. In contrast, the inner limb contains an inner ectoderm and an outer mesoderm. The outer limb becomes the chorion, and the inner limb becomes the amnion.
• As the amniotic folds grow centripetally, they fuse to form continuous membranes that enclose the embryo. By the end of the fourth day, the embryo is completely enclosed in a cavity called the amniotic cavity, which is bounded by the amnion. The space between the amnion and chorion is referred to as the sero-amniotic cavity or exocoel.
• The chorion serves several functions during embryonic development. The extraembryonic coelom, filled with fluid, provides protection to the developing embryo. It also provides space for the developing allantois, which plays a role in waste storage and respiration. The chorion combines with the allantois to act as a respiratory organ, facilitating gas exchange for the embryo.
• The amnion, on the other hand, is a sac-like structure surrounding the embryo. It contains amniotic fluid, which serves multiple functions. The amniotic fluid provides protection to the embryo, shielding it from mechanical shocks and desiccation. It also plays a crucial role when the egg is laid, creating an artificial aquatic environment that supports the embryo’s growth.
• In summary, the amnion and chorion are essential extra-embryonic structures in the development of the embryo. The amnion acts as a protective sac, providing a fluid-filled environment for the embryo’s growth and shielding it from external factors. The chorion, in conjunction with the allantois, facilitates respiration and provides space and protection for the developing embryo. Together, these structures contribute to the successful development and survival of the embryo during its early stages.

Yolk sac

• The yolk sac is an important structure that plays multiple functions during the development of the embryo. It first appears around 16 hours of incubation and develops from the splanchnopleure. The splanchnopleure consists of the endoderm and mesoderm layers. Instead of forming a closed gut, the splanchnopleure grows over the yolk, giving rise to the yolk sac.
• The yolk sac is located beneath the primitive gut, which is positioned above the yolk. This yolk region remains in contact with the midgut. Eventually, the yolk sac becomes connected to the midgut through an opening. This connection is facilitated by a narrow stalk known as the yolk stalk or umbilical stalk. Inside the yolk stalk, there is a narrow canal called the yolk duct.
• The yolk sac serves several functions throughout embryonic development. One of its primary roles is to digest the yolk, breaking it down into nutrients. These digested nutrients are then circulated through the bloodstream to nourish the developing embryo. As a result, the yolk sac is considered a nutritive organ, providing essential nutrients for the embryo’s growth and development.
• Additionally, during the early stages of development, the yolk sac also performs respiratory functions. It aids in the exchange of gases, facilitating the intake of oxygen and the removal of carbon dioxide. However, as the embryo continues to develop, other respiratory organs, such as the chorion and later the lungs, become the primary sites of gas exchange.
• In summary, the yolk sac is a vital structure in the early development of the embryo. It acts as a nutritive organ, digesting the yolk and providing essential nutrients for the growing embryo. Additionally, it plays a role in respiration during the initial stages of development. The yolk sac’s functions contribute to the overall nourishment and support of the embryo as it progresses through its developmental stages.

Allantois

• The allantois is an important structure that develops in the chick embryo from the ventral part of the caudal end of the hindgut. Its development begins around the third day of incubation. At this stage, a diverticulum forms on the floor of the hindgut, giving rise to the allantois. The allantois consists of an inner endoderm and an outer splanchnic layer of mesoderm.
• Initially, the allantois remains posterior to the yolk sac and rapidly expands, extending into the extraembryonic coelom. The distal end of the allantois enlarges to form an allantoic vesicle, while the proximal part becomes the allantoic stalk. Unlike the amnion and chorion, which develop externally, the allantois arises within the body of the embryo. The mesoderm of the chorion and the mesoderm of the allantois eventually unite, forming the chorioallantoic membrane. As the embryo grows, the allantoic stalk and yolk stalk come together, and their mesodermal layers unite, forming the umbilical stalk. The umbilical stalk is covered by the somatic umbilicus.
• The allantois performs several important functions during embryonic development. Firstly, it acts as a respiratory organ, being richly vascularized to facilitate gas exchange. Additionally, the allantois serves as a storage site for the embryo’s nitrogenous waste materials.
• Later in development, the allantois plays a role in calcium absorption. The allantoic circulation absorbs calcium from the eggshell, which is then utilized in the construction of bones in the young chick. This process gradually thins the eggshell, assisting in its rupture during hatching.
• In summary, the allantois is a structure that develops from the hindgut in the chick embryo. It functions as a respiratory organ, stores waste materials, and absorbs calcium from the eggshell. The allantois plays a crucial role in the embryonic development of the chick, contributing to its respiratory needs and overall growth and survival.

Summery of Membranes Of Chick

In avian embryos, extra-embryonic membranes play a crucial role in providing the necessary nutrients and support for the developing embryo. These membranes are external to the embryo’s body and serve various functions to ensure its survival and development.

1. Yolk sac: The yolk sac surrounds the yolk, which serves as a food source for the developing embryo. The yolk sac produces enzymes that convert the yolk material into a usable form for nourishment. Any unused yolk material in the yolk sac is absorbed into the abdomen of the hatched chicken, providing nutrition for the first few days after hatching until the chick learns to find food independently.
2. Amnion: The amnion forms a fluid-filled sac in which the embryo floats. This sac serves as a protective environment, cushioning the delicate embryo from potential harm due to external shocks or impacts.
3. Allantois: The allantois develops an extensive circulatory system connected to that of the embryo. It surrounds the embryo completely once fully developed. The allantois serves several important functions:

• Respiratory function: The allantois facilitates the exchange of oxygen and carbon dioxide for the developing embryo, as the embryo is unable to perform this function on its own. It oxygenates the blood and removes carbon dioxide.
• Excretory function: Metabolic waste products generated by the embryo are deposited in the allantoic cavity, and the allantois helps eliminate these waste materials.
• Digestive function: The allantois provides a means for the embryo to access the albumen (egg white) and the calcium present in the shell, aiding in the embryo’s nutritional needs.

4. Chorion: The chorion plays a vital role in connecting the inner shell membrane to the allantois. It assists the allantois in carrying out its functions effectively and provides support for its functioning.

Together, these extra-embryonic membranes enable the avian embryo to access nutrients, facilitate gas exchange, eliminate waste materials, and create a protected environment for its development. By relying on these specialized membranes, the avian embryo can obtain all the necessary resources for its growth and survival within the confines of the egg.

Daily embryonic development Summery

The daily embryonic development of a chick provides valuable insights for hatchery managers to assess the health and progress of the embryos. By observing the changes and milestones that occur each day, it becomes possible to identify any potential issues or causes for poor hatchability. Here is a summary of the key developments at different stages of daily embryonic growth:

• Day 1: Embryonic tissue becomes visible.
• Day 2: Tissue development becomes more apparent, and blood vessels start to form.
• Day 3: The heartbeat becomes detectable, and blood vessels become highly visible.
• Day 4: Pigmentation starts to appear in the eyes.
• Day 5: Elbows and knees begin to take shape.
• Day 6: The beak starts to form, and voluntary movements of the embryo begin.
• Day 7: Growth of the comb begins, and the egg tooth, a small protuberance on the beak, starts to appear.
• Day 8: Feather tracts become visible, and the upper and lower beak reach equal lengths.
• Day 9: The embryo starts to resemble a bird, and the mouth opening develops.
• Day 10: The egg tooth becomes prominent, and toe nails become visible.
• Day 11: The comb becomes serrated, and tail feathers start to emerge.
• Day 12: Toes fully form, and the first few feathers become visible.
• Day 13: Scales begin to appear, and the body is lightly covered with feathers.
• Day 14: The embryo turns its head toward the larger end of the egg.
• Day 15: The gut is drawn into the abdominal cavity.
• Day 16: Feathers cover the entire body, and the albumen (egg white) is nearly depleted.
• Day 17: Amniotic fluid decreases, and the head is positioned between the legs.
• Day 18: Embryonic growth is almost complete, with the yolk sac remaining outside the body. The head is under the right wing.
• Day 19: The yolk sac starts to retract into the body cavity, and the amniotic fluid is gone. The embryo occupies most of the space within the egg.
• Day 20: The yolk sac is completely drawn into the body, and the embryo transitions into a chick. It begins breathing with its lungs. Internal and external pipping occurs, indicating the start of the hatching process.

During hatching, the normal position of the chick is with the forepart of the body facing the larger end of the egg, the head positioned under the right wing, and the legs tucked up under the head. This position facilitates the chick’s emergence from the egg.

By closely monitoring the daily embryonic development and assessing the stage at which embryos may have died, hatchery managers can gain valuable insights into potential issues and make informed decisions to improve hatchability and overall chick health.

Structure of Egg of Hen

The egg of a hen has a distinct structure that supports the development and protection of the embryo. Here is an overview of the structure of a hen’s egg:

1. Size and Composition:
   • The egg is approximately 3.0 cm in diameter and is filled with the yolk.
   • The yolk is polylecithal, meaning it contains a large amount of yolk material.
   • The yolk consists of a central mass of white yolk surrounded by alternate concentric layers of yellow and white yolk.
   • Within the animal pole (upper portion of the yolk), there is a small cytoplasmic disc called the blastodisc, which contains a nucleus.
2. Membranes:
   • The yolk and blastodisc are enclosed by a plasma membrane and an outer vitelline membrane.
   • The vitelline membrane is a modified form of the zona radiata and has a double origin:
     • The inner layer is tough and composed of fibres produced in the ovary between the oocyte and follicle cells.
     • The outer layer is formed in the upper part of the fallopian tube.
3. Yolk Composition:
   • The yolk contains approximately 48.7% water, 32.6% phospholipids and fats, 16% proteins, 1% carbohydrates, and 1.1% other chemical molecules.
   • Proteins in the yolk are present as phosvitin and lipovitelline or livetin.
   • The fat in the yolk is primarily neutral fat (50%), along with phosphatids, cerebrosides, and cholesterol.
4. Albumen (Egg White):
   • The fertilized egg or zygote is surrounded by a layer of dense viscous albumen.
   • This dense albumen forms two twisted cords called chalazae, which are present at each end of the zygote.
   • The chalazae are formed as the egg rotates during its passage through the oviduct.
   • Surrounding the chalazae is a thick layer of watery albumen, which is secreted by the upper glandular walls or magnum of the oviduct.
   • The albumen serves as a source of nutrition for the developing embryo, acts as a water store, and provides a protective envelope against mechanical and chemical injuries.
   • The albumen also contains various enzymes, vitamins, pigments, and phosphorus.
5. Shell and Shell Membranes:
   • The isthmus part of the oviduct secretes two shell membranes made of tough keratin fibers that are closely applied to each other.
   • At the blunt end of the egg, the shell membranes are separated by an air space that forms after the egg is laid.
   • The shell is formed by the nidamental glands of the oviduct and is porous and calcareous.
   • The shell has numerous fine pores (approximately 7,000) filled with a protein similar to collagen.
   • These pores allow for the exchange of gases (oxygen and carbon dioxide) during the respiration of the developing embryo.
6. Incubation:
   • The egg is laid approximately 24 hours after fertilization, and further development occurs when the egg is incubated by the female.
   • Incubation must continue steadily for 21 days at a temperature of 103°F (39.4°C) for the embryo to fully develop.

Understanding the structure of a hen’s egg is essential for ensuring the proper development, nutrition, and protection of the growing embryo until hatching occurs.

Presumptive Fate Maps of Blastula

Presumptive fate maps of the blastula in chick embryos provide valuable insights into the future development of different cell populations. These maps are created using vital stains like carmine or carbon particles, as well as radioactive thymidine. The fate maps reveal the following information:

1. Area Opaca:
   • Blastomeres in the area opaca do not contribute to the formation of the embryo itself.
   • Instead, they give rise to the extra-embryonic membranes.
2. Area Pellucida:
   • The epiblast and hypoblast within the area pellucida exhibit distinct fates during embryonic development.
   • Fate mapping with tritiated thymidine reveals specific structures within the epiblast.
   • In the center of the area pellucida, a small region is designated as the presumptive notochord, which will develop into the notochord, an important axial structure.
   • Posterior to the notochord area, in the median plane, there is an elongated oval region representing the presumptive endoderm, which will give rise to the gut.
   • Toward the posterior edge of the area pellucida, the extra-embryonic endoderm is located. It will form the lining of the yolk sac.
   • Surrounding the presumptive notochord, endoderm, and extra-embryonic endoderm are various subdivisions of presumptive mesoderm:
     • Prechordal plate or head mesoderm
     • Mesodermal somites
     • Lateral plate mesoderm
     • Extra-embryonic mesoderm
3. Presumptive Ectoderm:
   • The anterior half of the epiblast corresponds to the presumptive ectoderm.
   • It includes a central region called the presumptive neural plate, which will develop into the neural tissue.
   • Anterior to the neural plate area is the presumptive embryonic epidermis, which will form the outermost layer of the embryo.
   • Surrounding the presumptive embryonic epidermis, in the form of a complete ring, is the extra-embryonic ectoderm.

By studying presumptive fate maps of the blastula, researchers gain a deeper understanding of the future differentiation and development of different cell lineages within the chick embryo. These maps provide insights into the formation of important structures such as the notochord, gut, mesodermal subdivisions, neural tissue, and various extra-embryonic tissues.

References

• http://extension.msstate.edu/content/stages-chick-embryo-development
• https://old.amu.ac.in/emp/studym/100004189.pdf
• https://www.yourarticlelibrary.com/biology/5-steps-involved-in-the-development-of-chick-explained/23153
• https://www.notesonzoology.com/vertebrates/chick/development-of-chick-with-diagram-vertebrates-chordata-zoology/8645
• https://w3.biosci.utexas.edu/experimentalembryology/ChickEmbryo.pdf
• http://www.macollege.in/app/webroot/uploads/department_materials/doc_139.pdf
• https://www.sites.ext.vt.edu/virtualfarm/poultry/poultry_development.html
• https://www.poultryhub.org/anatomy-and-physiology/body-systems/embryology-of-the-chicken
• https://www.notesonzoology.com/practical-zoology/developmental-stages-of-chick-embryo-zoology/3043