Eggs – Types, Structure, Egg Membranes, Examples

An egg is a biological structure which contains the female gamete and after fertilization it carries the developing embryo. It is an organic vessel that provides protection and nourishment to the embryo till hatching or birth. The major internal component of egg is yolk (vitellin) which acts as reserve food material and supplying energy for growth. Based on amount and distribution of yolk the eggs are classified into different types. Eggs with very little yolk are called microlecithal eggs as seen in mammals whereas eggs with large amount of yolk are called macrolecithal or megalecithal eggs as seen in birds and reptiles. The yolk may be evenly distributed or concentrated at one end which influences early cell division.

Egg membranes are the protective coverings present around the egg and they play an important role in survival of embryo. These membranes protect the egg from mechanical injury drying and also regulate fertilization by preventing entry of more than one sperm. These membranes are classified into primary secondary and tertiary membranes based on their origin. Primary membranes are secreted by the egg itself and are present just outside the egg surface such as vitelline membrane and zona pellucida. Secondary membranes are formed by follicle cells of ovary and examples include chorion and corona radiata. Tertiary membranes are secreted by oviduct or uterus and include jelly coat albumen shell membrane and calcareous shell.

In many terrestrial animals special type of egg known as cleidoic egg is present. This egg is completely enclosed by membranes and shell which prevent desiccation and provide protection. It allows reproduction away from water and is commonly found in reptiles and birds. The presence of albumen and porous shell helps in gas exchange while maintaining internal environment suitable for development of embryo.

Types of Eggs – Classification of Egg

A. On the basis of amount of yolk

Eggs are classified on the basis of amount of yolk present in them. The main types are as follows–

1. Alecithal egg

• These eggs are characterized by absence of yolk.
• The nourishment of embryo is obtained from mother.
• These eggs are found in eutherian mammals (placental mammals).

2. Microlecithal egg

• Microlecithal eggs contain very small or negligible amount of yolk.
• These eggs are also referred to as oligolecithal eggs.
• Examples are amphioxus and tunicates.

3. Mesolecithal egg

• Mesolecithal eggs possess moderate amount of yolk.
• The quantity of yolk is neither very less nor very high.
• These eggs are found in amphibians dipnoi and petromyzon.

4. Macrolecithal / Megalecithal / Polylecithal egg

• These eggs contain large amount of yolk.
• They are also known as megalecithal or polylecithal eggs.
• Reptiles birds and prototheria produce this type of egg.

B. On the basis of distribution of yolk

Eggs are also classified on the basis of distribution of yolk in the ooplasm. The types are as follows–

1. Isolecithal / Homolecithal egg

• In these eggs small amount of yolk is uniformly distributed in cytoplasm.
• These eggs are seen in animals having very less yolk.
• Examples include echinoderms amphioxus and some mammals.

2. Telolecithal egg

These eggs contain moderate to large amount of yolk. The yolk is unevenly distributed and concentrated towards vegetal pole.

Telolecithal eggs are of following types–

• i. Slightly telolecithal egg
  • These eggs contain small quantity of yolk distributed unevenly.
  • The vegetal pole contains more yolk than animal pole.
  • Fish eggs are examples.
• ii. Moderately telolecithal egg
  • These eggs contain moderate amount of yolk.
  • The nucleus is shifted towards animal hemisphere.
  • Amphibian eggs show this type.
• iii. Extremely telolecithal egg
  • These eggs contain very large amount of yolk.
  • The yolk occupies whole vegetal hemisphere and most of animal hemisphere.
  • Reptilian and avian eggs belong to this category.

3. Centrolecithal egg

• These eggs contain large amount of yolk in the centre.
• The nucleus is present in the centre of yolk mass.
• A thin layer of cytoplasm surrounds the yolk.
• These eggs are found in arthropods and some coelenterates.

C. On the basis of developmental potentialities

1. Mosaic egg

• In mosaic eggs developmental fate is predetermined.
• Removal of any part results in defective embryo.
• These eggs are found in annelids molluscs and ascidians.

2. Regulative egg

• In these eggs developmental fate is not fixed.
• Removal of blastomeres does not affect development.
• Most vertebrates and many invertebrates show regulative eggs.

D. On the basis of shell

1. Cleidoic egg

• These eggs are surrounded by thick hard shell.
• The shell is permeable to gases.
• These eggs contain large amount of yolk salts and water.
• Birds reptiles prototheria and some insects produce cleidoic eggs.

2. Non-cleidoic egg

• These eggs lack hard shell and possess soft membranes.
• They are found in viviparous animals and aquatic oviparous animals.
• Fish amphibians and mammals show non-cleidoic eggs.

Egg Envelops

Egg envelopes are protective coverings present outside the plasma membrane (oolemma) of ovum or oocyte. These are non-living structures and are formed at different stages during development of egg. Based on their origin these are classified into three main types.

Classification of Egg Envelopes

1. Primary Egg Membranes

These are secreted by the oocyte itself and are found very close to the egg surface.

• Vitelline membrane – It is a thin transparent membrane found in insects mollusks amphibians and birds. In birds it separates yolk from albumen.
• Zona pellucida – It is a thick non-cellular glycoprotein layer present in mammals. It helps in sperm binding and fertilization.
• Zona radiata – It is found in sharks some bony fishes and reptiles. It shows striations due to degenerated microvilli.
• Jelly envelope – It is a thick gelatinous layer present in echinoderms and some marine invertebrates.

2. Secondary Egg Membranes

These membranes are secreted by the follicle cells surrounding the oocyte.

• Chorion – It is a tough often chitinous shell present in insects ascidians and cyclostomes. It helps in protection and gaseous exchange.
• Corona radiata – It is present in mammals and consists of follicular cells attached to outer side of zona pellucida. These cells provide nourishment and protection to egg.

3. Tertiary Egg Membranes

These are secreted by oviduct or uterus after ovulation when egg passes through reproductive tract.

• Jelly coat – It is present in amphibians like frog. It absorbs water swells and provides buoyancy and protection.
• Albumen – It is found in birds and reptiles. It supplies water protein and acts as a protective medium.
• Shell membranes – These are fibrous membranes present below calcareous shell in birds egg and prevent bacterial entry.
• Calcareous shell – It is hard porous shell made of calcium carbonate present in birds and reptiles.
• Mermaid’s purse – It is horny egg case present in cartilaginous fishes like sharks and rays.

Functions of Egg Envelopes

• Protection – These envelopes protect embryo from mechanical injury desiccation and microbial infection.
• Prevention of polyspermy – Structural changes like cortical reaction prevents entry of more than one sperm.
• Buoyancy – Jelly coat in aquatic animals traps air and helps egg to float.
• Gaseous exchange – Porous shells allow oxygen and carbon dioxide exchange.
• Species recognition – Primary membranes contain specific receptors which allow only same species sperm to bind.

Examples of Eggs

Bird Eggs
These eggs are surrounded by hard calcareous shell.

• Ostrich – It produces the largest egg among living birds and weight may reach about 1.5 kg.
• Bee hummingbird and Vervain hummingbird – These lay the smallest bird eggs weighing about 0.3–0.5 g.
• Hen (Chicken) – It is the most commonly used egg for food and shell contains nearly 7000 pores.
• Bald eagle – Egg has strong calcareous shell.
• Cassowary – Eggs show rough and heavily pitted surface.
• Tinamou – Eggs are smooth and shiny in appearance.
• Guillemot – Eggs possess characteristic markings which help in identification on cliffs.
• Parasitic birds – Cuckoo and Brown-headed cowbird lay eggs in nest of other birds.

Reptile Eggs
These eggs are cleidoic and adapted for terrestrial condition.

• Snakes – Corn snake and African rock python lay soft leathery eggs.
• Turtles – Common snapping turtle and Chinese soft-shelled turtle lay eggs with leathery or calcareous shell.
• Lizards – Leopard gecko and Pseudomoia pagenstecheri lay shelled eggs.
• Dinosaurs – Eggs are known from fossilized calcified shells.

Amphibian Eggs
These eggs are non-cleidoic and usually laid in water.

• Frogs – Wood frog and Coast foam-nest tree frog lay gelatinous eggs.
• Salamanders – Eggs are similar to frog eggs and development occurs in water.
• Toads – They lay mesolecithal eggs containing moderate amount of yolk.

Fish Eggs
These eggs show variation in size structure and fertilization.

• Cartilaginous fishes – Whale shark produces very large egg about 30 cm long. Dog-fish skates and rays lay eggs enclosed in mermaid’s purse.
• Bony fishes – Trout Chinook salmon lungfish and coelacanth lay eggs of different sizes some reaching about 9 cm.
• Jawless fishes – Lamprey (Petromyzon) and hagfish lay small eggs.

Mammalian Eggs

• Monotremes – Platypus and Echidna lay macrolecithal eggs similar to reptiles.
• Placental mammals – Human mouse and rabbit produce microlecithal or alecithal eggs which are microscopic.

Invertebrate Eggs

• Insects –
  • Honey bee – Eggs appear like small grains of rice.
  • Butterfly (Monarch) – Eggs look pearl-like.
  • Stick and leaf insects – Eggs resemble plant seeds.
  • Praying mantis – Eggs are enclosed in ootheca.
  • Mosquito – Eggs are adapted to water and resist drying.
  • Silkworm – Eggs possess specific chorion loci used in genetic studies.
• Echinoderms – Sea urchin and starfish lay microlecithal eggs.
• Mollusks – Octopus snail and nudibranch lay eggs covered only by thin chorion.
• Crustaceans – Daphnia magna produces resting eggs enclosed in chitinous covering.

Structure of a typical Ovum

A typical ovum is a large non-motile female gamete which is structurally adapted for fertilization and early development. The different parts of ovum are described below.

1. Shape and Size
The ovum is generally spherical or ovoid in shape. In some insects it may be elongated or cylindrical. It is larger than somatic cells and sperm. The size varies from microscopic egg as in mouse to very large egg as in ostrich depending upon amount of yolk present.

2. Plasma Membrane (Plasmalemma)
It is the thin living membrane present just outside the cytoplasm of egg. Its structure is similar to plasma membrane of other cells. It regulates exchange of substances between egg and surrounding medium.

3. Cytoplasm (Ooplasm)
The cytoplasm of egg is called ooplasm. It is granular in nature and differentiated into two regions.

• Egg cortex (Exoplasm) –
  It is the outer peripheral layer of cytoplasm. It is viscous and gelatinous. It contains cortical granules pigment granules microtubules and microfilaments. Microvilli are present on this layer which help in transport.
• Endoplasm –
  It is the inner part of cytoplasm. It contains cell organelles enzymes histones tRNAs and yolk material.

4. Nucleus (Germinal Vesicle)
The nucleus of ovum is large and haploid. It contains female genetic material required for development. It is usually located near animal pole along polar axis.

5. Yolk (Deutoplasm)
Yolk is the reserve food material present in egg cytoplasm. It consists of proteins fats carbohydrates inorganic salts vitamins and water. It provides nourishment to developing embryo.

6. Cortical Granules
These are small spherical membrane bound bodies present in egg cortex. These are derived from Golgi complex. They contain mucopolysaccharides and take part in cortical reaction which prevents polyspermy.

7. Polarity of Egg
The contents of egg are unevenly distributed and two poles are formed.

• Animal pole – It contains nucleus and active cytoplasm.
• Vegetal pole – It contains maximum amount of yolk.

8. Egg Membranes (Egg Envelopes)
These are protective coverings present outside plasma membrane.

• Primary membranes – These are secreted by oocyte itself such as vitelline membrane and zona pellucida.
• Secondary membranes – These are secreted by follicle cells such as chorion and corona radiata.
• Tertiary membranes – These are secreted by oviduct such as albumen shell membranes calcareous shell and jelly coat.

Egg Membranes

Egg membranes are protective coverings present around the egg. These membranes protect the egg during development and are produced by different sources. Based on their origin egg membranes are classified into three types.

Classification of Egg Membranes

I. Primary Membranes

Primary membranes are secreted by the cytoplasm (ooplasm) of the egg. These form the innermost coverings and remain closely attached to egg surface.

• Plasma membrane – It is the living membrane covering the egg immediately. It is similar in structure to plasma membrane of other cells and is present in all eggs.
• Vitelline membrane – It lies outside the plasma membrane. It is thin transparent and composed of mucopolysaccharides and fibrous proteins. It is found in Amphioxus mollusks echinoderms amphibians and birds. The space between plasma membrane and vitelline membrane is called perivitelline space and it is filled with perivitelline fluid.
• Chorion (primary) – In some lower chordates like fishes (Styela) chorion is secreted by surface ooplasm and acts as primary membrane.
• Zona radiata – In shark (Scyllium canicula) and teleost fishes two primary membranes are present. Outer membrane is vitelline membrane and inner radiating membrane is zona radiata.
• Zona pellucida – In mammals the primary membrane is zona pellucida also called zona radiata. It shows striated appearance due to presence of microvilli from egg surface and macrovilli (desmosomes) from follicle cells.

II. Secondary Membranes

Secondary membranes are secreted by follicle cells of ovary. These membranes are usually tough and protective.

• Chorion – It is present in insects ascidians and cyclostomes (Myxine). It lies outside vitelline membrane. It is tough and impermeable and contains micropyles for entry of sperm.
• Corona radiata – It is present in mammalian eggs. It consists of follicle cells arranged radially around zona pellucida. It provides nourishment protection and support during fertilization.

III. Tertiary Membranes

Tertiary membranes are secreted by oviduct after ovulation. These are also called accessory membranes.

• Albumen (White of egg) – It is present in hen egg and lies outside vitelline membrane. It consists of three layers inner thin middle dense and outer thin albumen. It provides water protein and protection.
• Shell membrane – It is present outside albumen in birds egg. It is double layered and made of keratin. An air space is present between two layers at blunt end.
• Shell – It is outermost covering of birds egg. It is calcareous and made of calcium carbonate. It contains numerous pores for gaseous exchange.
• Jelly coat – It is present around amphibian eggs. It is gelatinous and protects egg from desiccation and predators.
• Mermaid’s purse – It is present in cartilaginous fishes like skates and rays. It is a hard horny egg case secreted by shell glands of oviduct. It has long twisted filaments for attachment to sea weeds.

These egg membranes together provide protection nourishment gaseous exchange and proper environment for embryonic development.

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