Which of the Following is Common to the Development of Both

Question

Which of the following is common to the development of both birds and mammals?
A)holoblastic cleavage
B)epiblast and hypoblast
C)trophoblast
D)yolk plug
E)grey crescent

Accepted Answer

Both birds and mammals develop from thick blastulae with distinct layers called the epiblast and hypoblast. This is unique to these two classes of animals and is not seen in other vertebrate groups. The epiblast is a layer of cells that gives rise to the embryo proper, while the hypoblast forms the yolk sac. The other choices are not shared between birds and mammals. Holoblastic cleavage, for example, is seen in some but not all groups of animals and refers to the way the fertilized egg divides into smaller cells. The trophoblast is the outer layer of cells that forms the placenta in mammals, but is not present in birds. Similarly, the yolk plug and grey crescent are both specific to amphibian development and are not relevant to birds or mammals.