Human Reproduction · Question 112

Cleavage in mammals is

1. holoblastic equal
2. holoblastic unequal
3. superficial
4. discoidal.

Answer

(2) holoblastic unequal

Solution

Mammalian eggs are alecithal (almost no yolk).

⇒ The whole zygote divides at each division, so cleavage is holoblastic (complete).

⇒ The blastomeres formed are not all of the same size (cleavage is slightly unequal and asynchronous), giving a morula and then a blastocyst with outer trophoblast and inner cell mass (as per NCERT).

∵ Superficial cleavage occurs in centrolecithal eggs (insects) and discoidal cleavage in megalecithal eggs (birds, reptiles).

∴ Cleavage in mammals is holoblastic unequal.

Answer: (2)