Question

Compare the size of a fertilized egg of frog with the size of its blastula and gastrula stages. Now choose the correct observation regarding size of them.
A. Zygote and blastula will be of same size, while gastrula will be larger
B. Progressive increase in size from zygote to blastula than gastrula is observed
C. All the three remain of same size
D. Blastula and gastrula will be larger/same size, while zygote will be smaller

Answer 1

Hint: After fertilization, a single-celled zygote is formed that undergoes cleavage to form a blastula. From a single-celled zygote, it divides to form a two-celled stage, which further divides to form a four celled stage. During this division, the size of cells at subsequent stages decreases which results in no change in the overall size of the structure. The size changes when the germinal layers are produced on the embryo.

Complete answer:
When the fertilized egg of a frog is compared with its blastula and gastrula stages, it is observed that the zygote and blastula will be of the same size, while the gastrula will be larger.

Once the fusion has occurred, a single-celled zygote is formed. After 2-3 hours, the mitotic division starts that first gives rise to a two-celled stage. The division of the two-celled stage produces a four celled structure and the division continues in the same manner. With each subsequent division, the size of the cells decreases. So, even though there is an increase in the number of cells in the blastula stage, it is of the same size as that of the zygote.

When the embryo gets surrounded by the germ layers, it is known as gastrula. The process of formation of gastrula is known as gastrulation. At the end of this process, the embryo is surrounded by 3 germ layers – ectoderm, mesoderm, and endoderm.

Hence, the correct answer is option (A).

Note: Due to the presence of germ layers, the gastrula is bigger than the zygote and blastula. Gastrulation occurs as a result of migration and rearrangement of the cells of the blastula. Gastrulation even involves the differentiation of cells to form the 3 germ layers. The organs of a frog are derived from the 3 germ layers.