Question

The eggs of eutherian mammals are
(A) Mesolecithal type
(B) Microlecithal type
(C) Telolecithal type
(D) Megalecithal type

Answer 1

Hint: Yolk of egg is used for supply of energy and synthesis of the products required for elaboration of the embryonic body. The yolk also has important influences on the size of the ovum, differentiation of ooplasm, patterning of cleavage, morphogenetic movements of blastomeres during gastrulation process, and the type of development whether direct or indirect.

Complete answer:
Eutherian mammals are also known as placental mammals. In eutherian mammals, the function of yolk is overtook by placenta. Here placenta provides nutrition to the growing embryo. So, there is no need for yolk.
In eutherian mammals eggs will form with no yolk or minimum amount of yolk. Such eggs are known as alecithal eggs.

In Microlecithal eggs the quantity of yolk is very less than the amount of cytoplasm. These eggs are very small in size. Some embryologists also described microlecithal eggs as alecithal eggs. The eggs of eutherian mammals are of this type. So, option (B) is correct.

> Option (A) is incorrect because in mesolecithal egg yolk is moderate in amount.
> Option (C) is incorrect because in telolecithal egg yolk becomes more abundant and tends to concentrate in one hemisphere of the egg.
> Option (D) is incorrect because in a megalecithal egg an enormous amount of yolk is present.

Note: Eutherian mothers carry their fetus within the uterus where they are nourished and protected by their mothers until an advanced stage is reached. This is made possible by the umbilical cord and placenta which helps in connecting the fetus to the uterus wall and enables nutrients and oxygen to get to the offspring as well as provides a means of releasing its waste. At the same time, the placenta functions as a barricade to keep the blood cells and other components of the immune systems of the mother and her fetus separate to prevent their destruction.