Types of Eggs and Egg Membranes

Eggs are fascinating structures found in various organisms across the animal kingdom. They serve as the starting point for new life, harboring the potential for development and growth. However, not all eggs are created equal. Different species exhibit diverse types of eggs, varying in size, shape, and the presence of yolk. Moreover, eggs are surrounded by protective membranes that play vital roles in ensuring the survival of the developing embryo. Understanding the types of eggs and their accompanying membranes provides insight into the remarkable diversity and adaptations seen in reproductive strategies. In this article, we delve into the different types of eggs and explore the fascinating world of egg membranes, uncovering their functions and significance in the journey of life.

Types of Eggs – Classification of Egg

A. On the Basis of the Amount of yolk

Eggs can be classified into different types based on the amount of yolk present in them. The following are the main types:

1. Alecithal Egg:
   • Alecithal eggs are characterized by the absence of yolk.
   • Examples of animals that produce alecithal eggs are eutherian mammals (placental mammals).
2. Microlecithal Egg:
   • Microlecithal eggs contain a small or negligible amount of yolk.
   • These eggs are also referred to as oligolecithal eggs.
   • Examples of animals that produce microlecithal eggs include amphioxus and tunicates.
3. Mesolecithal Egg:
   • Mesolecithal eggs have a moderate amount of yolk.
   • They are found in amphibians, dipnoi (lungfish), and petromyzon (lampreys).
   • The yolk content in these eggs is not high, hence the name mesolecithal.
4. Macrolecithal or Megalecithal or Polylecithal Egg:
   • Macrolecithal eggs contain a large amount of yolk.
   • They are also known as megalecithal or polylecithal eggs.
   • Reptiles, birds, and prototheria (monotremes) such as egg-laying mammals produce macrolecithal eggs.

B. On the Basis of the distribution of yolk

Eggs can be categorized into different types based on the distribution of yolk within them. The following are the main types:

1. Isolecithal or Homolecithal Egg:
   • In isolecithal eggs, a small amount of yolk is uniformly distributed throughout the ooplasm (cytoplasm).
   • This type of egg is observed in species with very little yolk.
   • Examples of isolecithal eggs can be found in echinoderms, amphioxus, and certain mammals.
2. Telolecithal Egg:
   • Telolecithal eggs contain a moderate or large quantity of yolk, and its distribution is not uniform.The yolk is concentrated more towards the vegetal pole (the lower pole of the egg).Telolecithal eggs can be further classified into three types:
   i. Slightly Telolecithal:
   • This type of egg contains a small quantity of yolk that is distributed unevenly.The highest concentration of yolk is found at the vegetal pole, while the animal pole has a lower concentration.Fish eggs are examples of slightly telolecithal eggs.
   ii. Moderately Telolecithal:
   • Moderately telolecithal eggs contain a moderate quantity of yolk that is distributed unevenly.The high concentration of yolk in the vegetal hemisphere causes the nucleus to be shifted towards the animal hemisphere.Amphibian eggs are an example of moderately telolecithal eggs.
   iii. Extremely Telolecithal:
   • In extremely telolecithal eggs, a heavy deposition of yolk occupies the entire vegetal hemisphere and a major portion of the animal hemisphere.
   • This uneven distribution of yolk displaces the ooplasm and nucleus towards the animal pole.
   • Reptilian and avian (bird) eggs are examples of extremely telolecithal eggs.
3. Centrolecithal Egg:
   • Centrolecithal eggs are found in many arthropods and some coelenterates.
   • These eggs are relatively large and elongated, containing a significant amount of yolk.
   • The nucleus is located at the geometric center of the yolk mass, surrounded by a small amount of cytoplasm.
   • Fine strands of cytoplasm extend from the peripheral layer to the zone occupied by the nucleus.
   • The surface of the yolk is covered by a thin layer of cytoplasm.

Additionally, eggs can also be classified based on their developmental potentialities:

a. Mosaic Egg:

• In mosaic eggs, every portion of the egg is predetermined with respect to its potential for further development.
• If a small portion of the egg is removed, a defective embryo is formed because the remaining portion cannot compensate for the lost part.
• Examples of animals with mosaic eggs include annelids (segmented worms), mollusks, and ascidians.

b. Regulative Egg:

• In regulative eggs, the developmental potentialities are not predetermined.
• Removing a small portion or even one or two early blastomeres (cells) does not affect normal development.
• The future developmental potentialities are flexible and can be regulated.
• Most vertebrates and many invertebrates have regulative eggs.

C. On the Basis of Shell

Eggs can be classified into different types based on the presence or characteristics of the outer shell. The two main types are:

a) Cleidoic Egg:

• Cleidoic eggs are characterized by having a thick and hard outermost shell.
• This hard shell is permeable to gases, allowing for gas exchange.
• Cleidoic eggs typically contain a large amount of yolk, salts, and water.
• These eggs are considered a terrestrial adaptation, providing protection and support for the developing embryo.
• Examples of animals that produce cleidoic eggs include birds, reptiles, prototheria mammals (such as monotremes like the platypus), and some insects.

b) Non-cleidoic Egg:

• Non-cleidoic eggs have soft egg membranes instead of a hard shell.
• These eggs are found in viviparous animals (animals that give birth to live young) and oviparous animals that lay eggs in water.
• Viviparous animals, such as mammals, do not have a hard shell around their embryos, as the developing young are nourished and protected internally.
• Oviparous animals that lay eggs in water, such as certain fish and amphibians, also have non-cleidoic eggs with soft membranes.

Egg Envelops

Egg envelopes refer to the membranes or coverings that surround the oocyte or ovum, present outside the plasma membrane. The term “envelopes” is now used instead of “membranes” to describe these structures. There are three types of egg envelopes based on their origin:

1. Primary Egg Envelopes:
   • Primary egg envelopes are secreted by the egg itself.
   • Examples of primary egg envelopes include the zona pellucida and zona reticulate.
   • The zona pellucida is a glycoprotein-rich layer found in mammals that surrounds the oocyte.
   • The zona reticulate is a network-like structure formed by follicle cells in the ovaries of certain animals.
2. Secondary Egg Envelopes:
   • Secondary egg envelopes are secreted by the ovary.
   • An example of a secondary egg envelope is the chorion found in insects.
   • The chorion is an outermost protective layer that surrounds the developing embryo in insect eggs.
3. Tertiary Egg Envelopes:
   • Tertiary egg envelopes are secreted by the oviduct.
   • Examples of tertiary egg envelopes include the jelly coat in frog eggs and the albumin, shell membrane, and shell in the eggs of hens (birds).
   • The jelly coat is a gelatinous layer that surrounds the frog egg, providing protection and support.
   • The albumin is the egg white, which serves as a source of nutrients for the developing embryo.
   • The shell membrane is a membrane layer surrounding the albumin, and the shell is the hard outer covering that provides further protection.

Examples of Eggs

1. Egg of Insect

The eggs of insects exhibit characteristics of being megalecithal polylecithal or centrolecithal. In these eggs, the cytoplasm is limited to the periphery, while the central region is occupied by yolk. There are two types of egg envelopes present in insect eggs:

1. Inner Egg Envelope – Vitelline Membrane (Primary Egg Membrane):
   • The vitelline membrane is the innermost egg envelope in insects.
   • It is considered the primary egg membrane and is secreted by the egg itself.
   • The vitelline membrane provides a protective barrier around the developing embryo.
2. Outer Egg Envelope – Chorion (Secondary Egg Membrane):
   • The chorion is the outermost egg envelope in insects.
   • It is the secondary egg membrane and is secreted by the ovary.
   • The chorion is characterized by being thick, hard, and often ornamented.
   • The ornamentation of the chorion can vary and is taxonomically significant in identifying different insect species.

Insect eggs do not have an egg membrane at a specific location, but there is a specialized region called the micropyle. The micropyle serves as an entry point for sperm to fertilize the egg. It is an important structure for reproductive processes in insects.

2. Egg of Frog

The eggs of frogs are characterized as mesolecithal and telolecithal. There are two types of egg membranes present on a frog egg:

1. Inner Egg Membrane – Vitelline Membrane (Primary Egg Membrane):
   • The vitelline membrane is the innermost egg membrane and is secreted by the egg cell itself.
   • It serves as the primary egg membrane and provides a protective covering for the developing embryo.
2. Outer Egg Membrane – Jelly Coat (Tertiary Egg Membrane):
   • The jelly coat is the outer egg membrane and is secreted by the oviduct.
   • It is a thick gelatinous layer surrounding the egg and plays several important roles.

Importance of the Jelly Coat in Frog Eggs:

• Buoyancy: The air bubbles present in the jelly coat provide buoyancy, helping the eggs float in the water.
• Mechanical Protection: The jelly coat protects the egg from external mechanical injuries.
• Predation Prevention: The tastelessness of the jelly coat prevents the eggs from being consumed by water snails, aquatic insects, fishes, and birds.
• Infection Prevention: The jelly coat acts as a barrier, preventing bacterial and fungal infections.
• UV Protection: The jelly coat, containing melanin, reflects sunlight and protects the eggs from harmful UV rays.
• Thermal Regulation: The jelly coat helps maintain optimal temperature for egg development.
• Egg Polarity: Frog eggs have two poles:
  • Animal Pole: This pole contains cytoplasm and the nucleus.
  • Vegetal Pole: This pole is rich in yolk, making it heavier and located at the lower side of the egg.
• Melanin Pigments: Melanin pigments present in the cytoplasm of the animal pole protect the egg from harmful radiation and aid in camouflage.
• Fertilization and Cleavage: Sperm enters the egg through the animal pole, determining the anterior part of the embryo. The sperm’s entrance path also influences the future polarity and the plane of cleavage in the egg. The first cleavage of the egg starts from a right angle to the sperm’s entrance path. Melanin granules move towards the sperm from the opposite direction, creating a clear region called the “Grey crescent,” which contributes to the formation of the dorsal lip of the blastopore in the future.

3. Egg of Chick

The eggs of chicks, which belong to birds, have the following characteristics:

1. Megalecithal or Polylecithal: Chick eggs have a large amount of yolk compared to the amount of cytoplasm present. The cytoplasm is concentrated in the form of a disc known as the Germinal disc.
2. Discoidal and Cleidoic: The cytoplasm of the chick egg is in the form of a disc, with yolk present beneath the germinal disc. The eggs are also cleidoic, meaning they have a hard and thick outer shell.
3. Yolk Types: The yolk in chick eggs is of two types based on color and content:
   • Yellow Yolk: This type of yolk contains more phospholipids.
   • White Yolk: This type of yolk is rich in protein but contains fewer phospholipids. The yolks are arranged in concentric layers, with the innermost and outermost layers consisting of white yolk.
4. Latebra and Isthmus of Pander: A stalk-like structure called the latebra is present just beneath the germinal disc. The upper part of the latebra is known as the Isthmus of Pander.
5. Envelopes: The primary envelope, known as the vitelline membrane, is found around the egg. All the remaining envelopes of the chick egg are secreted by the oviduct after fertilization. These additional envelopes are tertiary envelopes.
6. Albumin: The egg of a chick is filled with liquid albumin, commonly referred to as the “White of the egg.” It is produced by the isthmus part of the oviduct. Dense albumin forms coiled, cord-like structures called chalaza, which help keep the yolk suspended in the egg.
7. Shell Membranes: Two shell membranes made up of keratin are present just beneath the eggshell. These membranes provide additional protection.
8. Shell: The outermost part of the chick egg is the shell, which is composed of calcium carbonate (CaCO3). The shell is porous, allowing for the exchange of gases.

The oviduct of birds is differentiated into three parts:

• Anterior Part (Magnum): Fertilization of the egg occurs in this part. After fertilization, it secretes liquid albumin around the egg.
• Middle Part (Isthmus): This part secretes the keratin shell membranes around the egg.
• Posterior Part (Nidamental): This part secretes the shell made of CaCO3 around the egg.

These unique characteristics of chick eggs are essential for the development and protection of the growing embryo inside. The yolk provides nourishment, the various envelopes offer protection, and the shell allows for gas exchange while providing a sturdy outer covering.

4. Egg of Eutherian Mammals

Eutherian mammals, also known as placental mammals, have certain characteristics when it comes to their eggs:

1. Alecithal and Non-cleidoic: The eggs of eutherian mammals are small and do not contain a significant amount of yolk. They are also non-cleidoic, meaning they lack a hard, protective shell.
2. Microscopic Size: The eggs of eutherian mammals are microscopic, measuring around 0.1mm in diameter. They are much smaller compared to eggs of other animals.
3. Egg Envelopes: Two membranes or egg envelopes are present on the eutherian mammal egg:
   • Inner Envelope: Known as the Zona pellucida, this is the primary egg membrane produced by the egg itself. In some mammals, the Zona pellucida may appear linear and is referred to as the Zona radiata.
   • Outer Envelope: Called the Corona radiata, this envelope is formed by follicular cells of the ovary. The follicular cells are attached to the surface of the egg through Hyaluronic acid.
4. Sperm Entry: Mammalian sperm contain an enzyme called Hyaluronidase, which dissolves the Hyaluronic acid, allowing the sperm to penetrate and separate the corona radiata cells from the surface of the egg. This enzymatic action aids in the entry of sperm into the egg.
5. Minimal Yolk: The cytoplasm of eutherian mammal eggs contains very little or negligible amounts of yolk. These eggs rely on the placenta for nourishment and development during pregnancy.

Regarding ovulation, mammals can be classified into two types:

1. Induced or Reflex Ovulators: These animals have a fixed breeding period, and ovulation is induced by copulation. An example is the rabbit. The chances of fertilization are nearly 100% due to the close timing of copulation and ovulation.
2. Spontaneous Ovulators: In this case, ovulation is not dependent on copulation. The egg is released from the ovary into the oviduct after a fixed period. The chances of fertilization are relatively low in these animals since copulation does not trigger ovulation directly.

It’s important to note that the reproductive process in eutherian mammals primarily involves internal fertilization and subsequent embryonic development within the mother’s body, leading to live birth. The eggs play a crucial role in this reproductive process, allowing for the fertilization and development of the embryo.

Structure of a typical Ovum

The ovum, or the female gamete, plays a vital role in the process of development. It contains essential nutrients in the form of yolk, which sustains the embryo throughout its growth. The ovum serves three crucial functions:

1. Genetic Contribution: The ovum provides a haploid set of chromosomes to the future embryo, ensuring the inheritance of genetic information from the mother.
2. Cytoplasmic Contribution: The ovum contributes a substantial amount of cytoplasm to the zygote, which is formed upon fertilization. This cytoplasm contains various organelles and molecules necessary for early embryonic development.
3. Nutritional Support: The ovum supplies nourishment to the developing embryo, thanks to the stored yolk. This food source sustains the embryo during its initial stages of growth until it can establish a connection with the mother’s bloodstream or nutrient supply.

In summary, the ovum fulfills multiple roles in the reproductive process, including genetic transmission, cytoplasmic contribution, and providing essential nutrients for the development of the embryo.

Shape and Size

• Eggs exhibit various shapes and sizes depending on the species and the amount of yolk present. In most cases, eggs are spherical or ovoid in shape. However, certain animals, like insects, have elongated and cylindrical eggs.
• Compared to sperm cells and average somatic cells, eggs are generally larger in size. The size of a mature egg is influenced by the quantity of yolk it contains. The range in egg sizes across different species is significant. For instance, the smallest known egg is that of a mouse, measuring just 0.07mm in diameter. On the other end of the spectrum, birds lay larger eggs, with the ostrich producing the largest egg of all, boasting a diameter of approximately 85mm.
• Externally, eggs are covered by a plasma membrane, also known as the plasmalemma. Within this membrane lies the granular cytoplasm, which plays a crucial role in supporting the development and growth of the embryo contained within the egg.

Organisation of Egg Cytoplasm

• The cytoplasm of an egg cell, known as ooplasm, exhibits a specific organization. It is characterized by its granular nature and contains various components, including yolk, pigments, and cortical granules. The outermost layer of the ooplasm, called the egg cortex, is more viscous and gelatinous compared to the rest of the cytoplasm. The egg cortex is equipped with numerous microvilli and cortical granules.
• Microvilli are small projections that extend from the plasma membrane, formed by outpushings of the plasmalemma. These microvilli play a role in the transport of substances from the external environment into the ooplasm during egg development.
• Cortical granules are tiny spherical bodies found within the cytoplasm. They range in diameter from 0.8 µm to 2 µm and are enclosed by a membrane.
• These granules originate from the Golgi complex. They contain homogeneous and granular mucopolysaccharides, which are complex sugar molecules.
• Cortical granules are present in the eggs of various organisms, including sea urchins, frogs, fishes, bivalve molluscs, several annelids, and certain mammals.
• They contribute to various functions during fertilization and early embryonic development, such as modifying the egg’s external structure, preventing polyspermy (fertilization by multiple sperm), and initiating vital cellular processes.

Yolk

• Yolk, also known as deutoplasm, refers to the nutritive substances stored in the cytoplasm of an egg. This stored food serves as a source of nourishment for the developing embryo during its early stages. The formation of yolk is called vitellogenesis. Yolk is a complex substance that consists of proteins, fats, carbohydrates, inorganic salts, vitamins, enzymes, pigments, and water.
• The composition of yolk can vary. When the yolk contains a higher proportion of proteins than lipids, it is referred to as “protein yolk.” Conversely, if the yolk contains a higher amount of fats compared to proteins, it is known as “fatty yolk.” Most animal eggs contain both types of yolk.
• In certain eggs, such as those of frogs and chicks, the accumulation of yolk is concentrated in one region, making the yolk significantly heavier. These eggs are called telolecithal eggs. On the other hand, eggs with a lesser amount of yolk, such as those of Amphioxus (a type of fish) and humans, have a more uniform distribution of yolk throughout the cytoplasm. These eggs are known as isolecithal or homolecithal.
• Many species’ eggs contain pigment granules in the cytoplasm. These granules can exhibit various colors, including brown, black, red, yellow, green, or grey. However, since not all eggs possess pigment granules, their presence does not play a significant role in the development process.

Polarity

• Polarity refers to the asymmetrical distribution of constituents within an egg’s cytoplasm, resulting in the identification of two distinct poles: the animal pole and the vegetal pole. The cytoplasm is concentrated in the upper portion, known as the animal hemisphere, while the yolk material is concentrated in the lower portion, called the vegetal hemisphere. The axis passing through these poles is known as the polar axis.
• The nucleus of the egg is typically located within the polar axis, slightly towards the animal pole. The yolk content gradually decreases as one moves from the animal pole towards the vegetal pole, creating a gradient. Along the polar axis, there is also a gradient in metabolic processes, with the highest metabolic activity occurring at the animal pole and gradually diminishing towards the vegetal pole.
• This polarity in the distribution of cytoplasm, yolk, and metabolic activity is crucial for the subsequent development of the embryo. It sets the stage for specific cellular and molecular events that occur during early embryonic development, influencing cell differentiation, pattern formation, and the establishment of body axes. The polar axis and the inherent polarity of the egg contribute to the overall organization and development of the embryo.

Egg Membranes

Egg membranes play a crucial role in protecting the eggs during development. These membranes can be produced by different sources, including the egg itself, the follicle cells of the ovary, or the genital ducts (oviduct) of the female.

Based on their origin, egg membranes are classified into three types: primary membranes, secondary membranes, and tertiary membranes.

1. Primary membranes: Primary membranes are produced by the egg itself. They form the innermost layer of egg membranes. Examples of primary membranes include the vitelline membrane, which is a primary egg membrane found in many species.
2. Secondary membranes: Secondary membranes are produced by the follicle cells of the ovary. These membranes surround and protect the egg, providing an additional layer of defense. An example of a secondary membrane is the chorion found in insects.
3. Tertiary membrane: Tertiary membranes are produced by the genital ducts or oviducts of the female. These membranes are typically formed after fertilization and serve to further protect the developing embryo. Examples of tertiary membranes include the jelly coat found in frog eggs and the albumin, shell membrane, and shell of hen eggs.

Each type of egg membrane has its own specific functions and characteristics, contributing to the overall protection and development of the embryo. These membranes help prevent mechanical injury, provide a barrier against external pathogens, regulate gas exchange, and provide structural support. The presence of different layers of egg membranes ensures the eggs are well-protected and have the necessary environment for successful embryonic development.

I. Primary membranes

Primary membranes are the first layer of membranes that are secreted by the cytoplasm (ooplasm) of the egg. These membranes are closely attached to the surface of the egg and provide important functions in protecting and supporting the developing embryo. Different types of primary membranes exist in various animals:

1. Plasma Membrane: This membrane covers the egg immediately and is present in the structure of all eggs. It resembles the plasma membrane of a cell and serves as the outermost layer of the egg.
2. Vitelline Membrane: The vitelline membrane is closely attached to the plasma membrane of the egg. It is commonly found in the eggs of animals such as Amphioxus, mollusks, echinoderms, amphibians, and birds. The vitelline membrane is thin, transparent, and composed of mucopolysaccharides and fibrous proteins. The space between the vitelline membrane and the plasma membrane is called the perivitelline space, which is filled with a fluid known as perivitelline fluid.
3. Chorion: The chorion is found in the eggs of lower chordates, such as fishes (e.g., styela). It is a product of the surface ooplasm and acts as a primary membrane in these organisms.
4. Zona Radiata: In some species, such as the shark Scyllium canicula and teleost fishes, there are primary membranes produced by the surface ooplasm. The outer membrane is called the vitelline membrane, while the inner membrane has a radiating appearance and is known as the zona radiata.
5. Zona Pellucida: Mammalian eggs are surrounded by a primary membrane called the zona pellucida, which is also referred to as the zona radiata. It is named as such because it exhibits a striated appearance under a microscope. The striations are due to the presence of microvilli and macrovilli (desmosomes) within the zona pellucida. The microvilli are produced by the egg’s surface, while the macrovilli are produced by follicle cells and protrude into the zona pellucida.

These primary membranes provide physical protection, regulate the exchange of substances, and contribute to the overall structure of the egg. They play important roles in fertilization, embryonic development, and interactions between the egg and its environment.

II. Secondary Membranes

Secondary membranes are a type of membrane that is produced by the follicle cells, which are cells found around the developing oocytes (egg cells) in the ovary. These membranes are typically tough and impermeable, providing additional protection to the developing embryo. There are two main types of secondary membranes:

• Chorion: The chorion is a common outer covering found in the eggs of insects, ascidians (sea squirts), and cyclostomes (e.g., Myxine, commonly known as hagfish). It is located outside the vitelline membrane. The chorion is characterized by its toughness and impermeability. It often has one or more openings called micropyles, which allow sperm to enter the egg for fertilization. The chorion plays a crucial role in shielding the developing embryo from external threats.
• Corona Radiata: The corona radiata is a secondary membrane specifically found in mammalian eggs. It is formed by a layer of follicle cells that surround the zona pellucida, which is the primary membrane in mammalian eggs. The corona radiata consists of follicle cells that are radially arranged around the zona pellucida. These cells provide additional support and protection to the egg, particularly during the process of fertilization.

Secondary membranes, such as the chorion and corona radiata, contribute to the overall structure and integrity of the egg. They serve as barriers against mechanical damage, protect against harmful substances, and facilitate interactions between the egg and its surrounding environment. These membranes play important roles in reproductive processes and the successful development of the embryo.

III. Tertiary Membranes

Tertiary membranes, also known as accessory membranes, are a type of membrane produced by the oviduct (the reproductive tract) in certain animals. These membranes provide additional protection and support to the developing embryo. Some examples of tertiary membranes include:

1. White Albumen: Found in the eggs of hens, the white albumen is located outside the vitelline membrane. It consists of three layers: an inner less dense albumen, a middle dense albumen, and an outer less dense albumen. The albumen is primarily composed of water and protein, providing nourishment and moisture to the developing embryo.
2. Shell Membrane: The shell membrane is formed around the albumen in the eggs of hens. It is a double membrane consisting of two closely adhering layers. Between these layers, there is an air space present at the blunt end of the egg. The shell membrane is made of keratin, which adds strength and protection to the egg.
3. Shell: The shell is the outermost covering of eggs laid by land animals. In the case of birds, such as hens, the shell is primarily composed of calcium carbonate. It can be white or brown in color and contains numerous tiny pores, ranging from 0.04 to 0.05 mm in diameter. These pores allow for gas exchange between the developing embryo and the external environment. The shell also provides a physical barrier against external threats.
4. Jelly Coat: Amphibian eggs are surrounded by a gelatinous covering called the jelly coat. This jelly-like substance provides protection to the developing embryo and helps maintain moisture. It also serves as a barrier against dehydration and predators.
5. Mermaid’s Purse: The mermaid’s purse is an egg case found in certain cartilaginous fishes, such as skates and rays. It is a protective, hard shell secreted by the shell glands present in the oviduct. The shape of the mermaid’s purse can vary among different species, but it is often rectangular. The corners of the shell are drawn out into four long, twisted elastic filaments, which serve to attach the eggs to underwater structures, such as seaweed. The mermaid’s purse provides physical protection to the developing embryos.

Tertiary membranes play crucial roles in safeguarding the developing embryos by providing support, nourishment, and protection against environmental factors.

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