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Last updated date: 23rd Nov 2023
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MVSAT Dec 2023

In a mesolecithal egg one would expect
A. Meroblastic cleavage
B. Unequal holoblastic cleavage
C. Equal holoblastic cleavage
D. None of these

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Hint: The egg is a vessel containing organic zygote in which the embryo develops until it can survive on its own, at which point the animal hatches. The egg is the product of egg cell fertilisation.

Complete Answer:
There are three types of eggs, depending on the proportion of the yolk to the ovum cytoplasm.
- Microlecithal egg: The amount of yolk in microlecithal eggs is much less than the amount of cytoplasm. These eggs are tiny in size.

- Mesolecithal eggs: Here the amount of yolk is moderate and these eggs are called mesolecithal or megalecithal (i.e. median yolk). The distribution of the yolk is obviously uneven. The shark eggs, ganoid fish, dipnoi and several amphibians are of this kind.

- Macrolecithal or polylecithal eggs: Macrolecithal eggs contain huge quantities of yolk, and the yolk is several times larger than the cytoplasm. These eggs may be huge or tiny. This type includes teleost fish eggs, gymnophiona (legless amphibians), reptiles , birds and monotremes (egg laying mammals). Cleavage, a sequence of mitotic divisions, with immense divisions. Amount of egg cytoplasm is divided into several smaller nucleated cells. These cleavage-stage cells are called blastomeres

- Holoblastic or total cleavage: When the cleavage furrows separate the whole egg. It may be:
(I) Equal: When the furrow of the cleavage cuts the egg into two equal cells. They can be radially symmetrical, bilateral, symmetrical, irregular or spirally symmetrical.
(II) Inequal: When the resulting blastomeres become inconsistent in size.

- Meroblastic cleavage: It is called meroblastic cleavage when segmentation occurs only in a small portion of the egg that results in blastoderm formation. Mesolecithal eggs have unequal holoblastic cleavage.

Thus the correct answer is option(B).

Note: Most arthropods, vertebrates (excluding live-bearing mammals) and molluscs lay eggs, while some, such as scorpions, do not.