Question

The eggs of some mammals have more yolk. They are:
A. Aquatic mammals
B. Metatherians
C. Protherians
D. Eutherians

Answer 1

After the process of fertilization the zygote is formed. The zygote then undergoes numerous mitotic divisions to form an embryo in which the cell number increases but cell enlargement or growth does not occur. This process is also termed as cleavage.

Complete answer:
After the process of cleavage, the extra embryonic layers start developing and out of them, one is the formation of yolk sac. The type of cleavage decides the amount of yolk present in the cells. The eggs are divided into three major groups on the basis of the amount of yolk present in them. These are as follows:
Alecithal egg: this type of eggs contains no yolk. Example: Eutherian mammals.
Microlecithal egg: when the egg contains very less or negligible amount of yolk it is said to be a microlecithal egg. Example: Amphioxus, Tunicates.
Mesolecithal egg: if the amount of yolk present in the egg is not high, but moderate, then it is called a mesolecithal egg. Example: some amphibians like Dipnoi and Petromyzon.
Macrolecithal or Megalecithal egg: when there is presence of large amount of egg yolk in the egg, then it is termed as megalecithal or macrolecithal egg. Example: Reptiles, Birds, Prototherians etc.

Therefore, the correct answer for this question is option C- Protherians.

Note: The type of cleavage is decided by the amount of yolk in the eggs. In case of placental mammals where nourishment is provided by the mother’s body, the eggs have a very less yolk and undergo holoblastic cleavage. On the other hand the other species, such as birds, which have a lot of yolk in the egg to nourish the embryo during development, undergo meroblastic cleavage.