Question

Why is the human ovum called alecithal?

Answer 1

Hint: The egg is primarily divided into four forms, depending on the amount of yolk present. Nutrients are contained in the form of yolk or deutoplasm inside the cytoplasm of the egg. The embryo employs yolk as the stored food for its early development.

Complete answer:
Alecithal eggs are eggs that have no yolk present. E.g. The Eutherian mammalian eggs. One that has a negligible amount of yolk or no yolk is an alecithal egg. The yolk provides nutrients required for the embryo to develop and thus, its presence is essential in oviparous species. In humans, however, the developing embryo remains attached to the mother and obtains nutrients via the placenta from the mother. Their shell, therefore, has a marginal amount of yolk and is alecithal. Eggs are divided into three forms based on the concept of the amount of yolk present in them.
a) Microlecithal Egg: These are eggs that contain small or negligible amounts of the yolk. E.g. Amphioxus, Tunicates.
b) Mesolecithal Egg: The amount of yolk present in amphibian, Dipnoi and Petromyzon is moderate and not large. Thus, these eggs are often referred to as mesolecithal eggs.
c) Macrolecithal or megalecithal or polylecithal egg: An egg is said to be a macrolecithal or megalecithal egg if the egg contains an outsized quantity of yolk. Eg. Reptiles, Mice, Prototheria mammal-laying (Monotremata).

Note: Chick eggs are mesolecithal or polylecithal. Beneath the blastoderm yolk, there are two forms of yolk on the concept of colour and substance. Yellow yolk has more phospholipids. The white yolk is high in protein and there are fewer quantities of phospholipids. In alternate and condensed layers, both kinds of yolks are organised.