Question

Assertion: In the morula stage, the cell divides without an increase in size.
Reason: Zona pellucida remains till cleavage is finished.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
C. The assertion is correct but Reason is incorrect
D. Both Assertion and Reason are incorrect

Answer 1

The development of the embryo is a multistage process where it initially divides into two cells, to four cells, to eight cells, and then to a 16 cell stage which is known as the morula. The morula then divides into a 32 cell cluster where there is only an increase in cell number. After this stage, the zona pellucida cleaves to form the blastocyst and then to a blastula.

Complete answer: The fertilization of the zygotes occurs in the fallopian tube, where the sperm enters into the ovum through the zona pellucida. The zona pellucida is a thick glycoprotein layer that envelops the ovum. The zona pellucida also binds with the spermatozoa and enables the gamete to move inside the ovum and fuse. Once the zygote is formed, it begins to divide initially into two cells. This then progresses into a four-cell cluster, on to an eight-cell cluster, and then to a 16 cell clump, known as the morula or morula stage. After the 16 cell stage, the cells divide into a 32 cell stage, where the cell does not increase in size but forms an inner cell mass. At this stage, cells even though divide does not increase in size because cells bind firmly to each other by a process called compaction. Even during this stage, the zona pellucida is still intact. After the 32 cell stage, the cell further divides into a cell mass of around 70 to 100 cells, where the zona pellucida is cleaved and a ring of undifferentiated cells called the trophoblast to surround the inner cell mass. This stage is known as the blastocyst stage and the cell mass is known as the blastocoel. The blastocoel begins to further divide and the zona pellucida is completely cleaved and the trophoblast starts to differentiate.
Hence, the correct answer is option B.

Note: The 16th cell stage is known as the morula stage and the zona pellucida remains intact.
The cells from the 16th cell stage to the 32nd cell stage, divide but do not increase in size because they form an inner cellular clump. The zona pellucida only cleaves and degenerates for the trophoblast to form which is crucial for implantation and is known as zona hatching. The zona hatching is not directly related to preventing an increase in the size of the cells within the morula.