In this article we will discuss about the embryonic development of egg and yolk in insects and birds.
The eggs are generally spherical but eggs of insects are elongated and cylindrical. The eggs of some polychaetes, fishes and birds are oval in shape. The egg cells are covered by plasma membrane which is secreted by egg cytoplasm (ooplasm).
The another thin and transparent membrane, called vitelline membrane, remains attached to the outside of the plasma membrane which is found in insects, molluscs, echinoderms, amphibians and birds. This membrane after fertilization is called fertilization membrane. Another membranes, such as chorion, zona radiata, zona pellucida, jelly coat are found in different groups of animals.
The egg cytoplasm (ooplasm) contains yolk, pigments, cortical granules, etc. The cytoplasm is not homogenous mass and its substances are not equally distributed.
Yolk is a nutritive substance consisting largely of proteins and fats. It is usually produced within egg cell but in platyhelminthes it is produced by vitelline glands. The amount of yolk tends to obstruct the cleavage and affects the cleavage pattern of the developing embryos. It is also related to the pattern of larval development in the larva producing individuals.
Bilateria, where the embryos develop from less amount of yolk-contain eggs, become planktotrophic larvae because they feed on planktons of other organisms.
The larvae which develop from the heavily yolk-laden eggs, become lecithotrophic larvae because they depend on the reserve yolk of the ovum (e.g., nereids, eunicids, some bivalves, etc.). The planktonic larvae are seen in echiurans, phyllodocids, serpulids, some gastropods, etc.
The eggs are classified on the basis of the amount of yolk present in the cytoplasm and the distribution of yolk.
A. Egg type based on the amount of yolk:
When the eggs contain very small amount of yolk, they are called microlecithal eggs. The eggs are small-sized and found in Hydra, sea urchins, amphioxus (Branchiostoma), tunicates and eutherian mammals. Kent (1969) has described the microlecithal eggs as alecithal eggs but Balinsky (1981) used the term oligolecithal eggs instead of microlecithal eggs.
When the eggs contain a moderate amount of yolk, they are called mesolecithal eggs and are found in certain annelid worms, certain molluscs, petromyzons, lungfishes and amphibians.
Megalecithal (=Macrolecithal) eggs:
When the eggs contain a large amount of yolk, called the megalecithal eggs and are found in primitive families of aquatic oligochaetes, cephalopods, fishes, reptiles, birds and prototherian mammals.
B. Egg type based on the location of yolk within cell:
1. Homolecithal or Isolecithal eggs:
In microlecithal eggs, the amount of yolk is very small. It is found scattered uniformly throughout the egg cytoplasm and evenly distributed in the ooplasm. This type of eggs is seen in echinoderms, some molluscs, ascidians, cephalochordates and eutherian mammals.
2. Telolecithal eggs:
When the yolk of mesolecithal and macrolecithal eggs is concentrated at one end (the vegetal pole) of the eggs is called telolecithal eggs. The yolk due to its gravity is concentrated at one pole than the other.
The yolk where the concentration is greatest is called the vegetal pole and the pole where the yolk concentration is smallest and the nucleus is situated, is called animal pole. It is opposite to the vegetal pole. The telolecithal eggs may be moderately telolecithal (e.g., oligochaetes, amphibians) and highly telolecithal (e.g., cephalopods, fishes, reptiles, birds and prototherians).
3. Centrolecithal eggs:
When the yolk is aggregated at the centre of the egg and the active cytoplasm remains in the peripheral layer around the yolk. These eggs are called centrolecithal eggs and are found in some Hydrozoa and in some insects. The yolk-free cytoplasm is found in many insects and the cleavage is total in Collembola.
Polarity of the eggs:
All types of eggs have the polarity but it is highly marked in telolecithal eggs. The two poles are designated as vegetal pole and animal pole. An imaginary line which passes through the centre of the animal pole to that of the vegetal pole is called animal-vegetal axis or median axis or polar axis.
The animal pole indicates the future anterior region of the developing embryos or the future anterior or head region of the animals and the vegetal pole develops as the posterior region of the animals.