The yolk content of the egg and its distribution varies in different eggs. This is used as a criterion to classify the eggs into several types.
A. Classification based on amount of yolk:
1. Alecithal. Here there is no yolk in the egg. Eg., eutherian mammals.
2. Microlecithal. Eggs have a small amount of yolks, eg., amphioxus.
3. Mesolecithal. Eggs have a moderate amount of reserve food material eg., amphibians like salamander, frog, toads etc.
4. Macrolecithal or polylecithal. These eggs have a large amount of yolk and also a little quantity of yolk free cytoplasm. Eg. Reptiles, brids and prototherian mammals.
B. Classification based on distribution of yolk:
1. Homolecithal or Isolecithal. In this type, the quantity of yolk is very less and it is uniformly distributed all over the egg cytoplasm. Eg., eggs of echinoderms.
2. Heterolecithal. In these types of eggs, yolk is not evenly distributed in cytoplasm. Based on the location and area it occupies, heteroletichal eggs may be of following types.
Here the yolk is concentrated in one half of the egg to form the vegetal pole and the yolk free portion is known as the animal pole. Eg., amphibians, reptiles, birds and prototherian mammals.
The quantity of the yolk is very large 5nd it occupies almost the entire interior of the egg except for a small disk shaped portion of the cytoplasm. The cytoplasm contains the nucleus eg., fishes, reptiles etc.
Here the yolk occupies exactly the central portion of the egg while the cytoplasm is peripheral. Such eggs are found in insects.