The proportion of proteins and neutral fats varies and the yolk is accordingly known as 'protein yolk' or 'fatty yolk.' Depending upon the amount of yolk and its nature of distribution the eggs are grouped into four types.
In this type Yolk granules are fine, evenly distributed and the amount of yolk is small. Such eggs are known as oligolecithal and are found in echinoderms, lower chordates.
In Meiolecithal type of egg, area of the egg except a small portion for the cytoplasm will be yolk. The yolk will be concentrated in one hemisphere. Eggs of Fish & frogs are examples for Meiolecithal egg.
The Yolk is found in excess and is in the form of large platelet like granules. The yolk is unequally distributed, more densely packed in the posterior half. Such eggs are found in sharks, amphibians, birds, cyclostomes, elasmorbranchs, ganoids and ling fishes.
In the eggs of arthropods, especially insect yolk is centered in the interior and the cytoplasm forms a thin coat on the external surface.
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