Ques. 1AIPMT 2000
Cleavage in mammals is
- holoblastic equal
- holoblastic unequal
- superficial
- discoidal.
Answer
(2) holoblastic unequal
Solution
Mammalian eggs are alecithal (almost no yolk), so the whole zygote divides at each division and cleavage is holoblastic (complete).
The blastomeres formed are not all of the same size (cleavage is slightly unequal and asynchronous); as per NCERT they give a morula and then a blastocyst with outer trophoblast and inner cell mass.
Superficial cleavage occurs in centrolecithal eggs (insects) and discoidal cleavage in megalecithal eggs (birds, reptiles).
∴ Cleavage in mammals is holoblastic unequal.
Answer: (2)