Question

In mesolecithal eggs, the cleavage is
(a)Equal, holoblastic
(b)Unequal, holoblastic
(c)Meroblastic
(d)Discoidal

Answer 1

Hint: The mesolecithal eggs are eggs that have mediate content of yolk sac and the yolk is mainly in one part of the egg known as the vegetal pole. The cell nucleus and the majority of the cytoplasm are in the animal pole.

Complete answer:
There are three types of eggs based on the eggs of chordates and they are microlecithal which are simple eggs with little yolk, mesolecithal which have uneven cell cleavage majority in the cytoplasm rich animal pole, and lastly macrolecithal which have a largely concentrated yolk. The cleavage in eggs can be classified as holoblastic which is complete cleavage or meroblastic that is partial cleavage depending on the amount of yolk sac. Ignoring the large concentration of yolk mainly four cleavage types can be seen in microlecithal and mesolecithal cells and they are radial holoblastic, rotational holoblastic, bilateral holoblastic, and spiral holoblastic cleavage.

So, the correct answer is 'Unequal, holoblastic'.

Note: Birds such as hen’s egg show meroblastic cleavage. The cleavage furrows when they occur, do not follow down by way of the mass of yolk so each of the cells produced in the premature stages has no direct contact with yolk at the bottom of the cell but is bound on top and sides by a plasma membrane. In other words, in such cases, the blastomeres are still partially connected. Animals like cephalopod mollusks that are sea animals like octopus, squid, and cuttlefish show meroblastic bilateral cleavage. Humans show holoblastic cleavage and the nourishment is provided by the mother's body. The eggs have a very small amount of yolk and are microlecithal.