Q.22 The egg of birds and fishes have only one small area which is free of yolk and where division takes place.
Such an egg is known as:
1. Telolecithal
2. Centrolecithal
3. Isolecithal
4. Mesolecithal
Bird & Fish Eggs: Telolecithal Type with Small Cleavage Area
Birds and fish eggs feature heavy yolk concentrated at one pole, leaving a small disc-shaped animal pole for meroblastic cleavage. The correct answer is 1. Telolecithal.
Correct Answer
Telolecithal eggs have yolk disproportionately concentrated at the vegetal pole, displacing cytoplasm and nucleus to a small animal pole disc where division occurs—typical of fish (moderate telolecithal) and birds/reptiles (extreme telolecithal). This enables discoidal meroblastic cleavage atop the yolk mass.
Option Explanations
1. Telolecithal (Correct)
Yolk mass occupies vegetal hemisphere, leaving minimal active cytoplasm at animal pole for cleavage in macrolecithal eggs of vertebrates like fish, amphibians, reptiles, birds.
2. Centrolecithal (Incorrect)
Central yolk surrounded by peripheral cytoplasm (insect eggs); superficial cleavage around yolk periphery, not matching bird/fish polarized distribution.
3. Isolecithal (Incorrect)
Evenly distributed minimal yolk throughout cytoplasm (echinoderms, mammals); permits holoblastic equal cleavage, opposite of yolk-concentrated eggs.
4. Mesolecithal (Incorrect)
Moderate uneven yolk (amphibian eggs); larger active region than fish/birds but still telolecithal subtype, not distinctly separate category here.
