What is the correct observation regarding the size of egg, blastula, and gastrula of a frog?
a. There is a progressive increase in size from zygote to blastula to gastrula
b. all the three are of the same size
c. Zygote is small while blastula and gastrula are larger
d. Gastrula is larger while zygote and blastula are of the same size

Hint: Frog egg cell volume is 1.6 million times larger than the normal frog body cell, these cells are converted into tadpole during egg development. Not much variation can be seen in the blastula, zygote in comparison.

Complete answer:
Not much variation in size can be seen in the zygote and blastula cell size.
As they both are covered with a cell lining known as zona pellucida, the outer covering of the cell.
Zona pellucida has the property of inhibiting the cell growth formation.
Due to which size increase in the zygote and blastula cannot be seen, it is observed that they are of the same size.
When the cleavage process occurs in the cell after the third stage cleavage, where the eight-cell stage is formed, the blastula is formed.
When gastrulation takes place zona pellucida is destroyed or degrades (morula) which causes an increase in the size of the gastrula.
Which makes gastrula cell size much larger, as the cells start dividing accumulating as a layer after layer.
Hence, the size of the gastrula becomes larger than the zygote and blastula.

Hence, the correct answer is option (D).

Note: Frog embryology follows the development of the egg where after meiosis two egg cells become ready for fertilisation and after fertilisation the egg cell consists of the two small second polar body, grey crescent in the middle. After fertilisation formation of blastula, cleavage, gastrula, differentiation, growth takes place.