Question

Mass of cells formed during early embryonic development of an animal is?
A. Morula
B. Blastula
C. Amphiblastula
D. Gastrula

Answer 1

Hint: Fertilization is an event which involves various processes like chemotactic movements of gametes, the fusion of gametes and syngamy. This event leads to the formation of a two-celled zygote and this zygote further develops into the embryo. The embryo develops into a foetus.

Complete answer: When sperms fuse with the vitelline membrane, the oocyte completes its meiosis-II. After the completion of meiosis, the formation of the second polar body and haploid egg nucleus occurs. Sperm nucleus and egg nuclei fuse to form a diploid zygote. Initially, this zygote divides into a two-celled stage i.e. formation of two blastomeres. This division occurs through the meridional plane. During the second division. The plane of cleavage passes perpendicular to the first division and four cells are formed. All cleavage divisions occur through mitosis and lead to the formation of two blastomeres. During cleavage division, the size of blastomeres remains the same and only the genetic material and the number of cells increases during every division. The zona pellucida is remaining intact during cleavage division and that’s why the size of blastomere remains decreasing and mass of cytoplasm is also decreasing. The amount and distribution of yolk determine the plane of cleavage. Cleavage results in the formation of a solid ball of cells. This solid ball consists of 8-16 cells. Zona pellucida is still attached to the outer cover. The morula undergoes compaction i.e. it becomes denser. The cells present in the outer or peripheral region are smaller or flat with the presence of tight junctions while the cells present in inner cell mass consist of rounded and slightly large cells having gap junctions.
Hence, the correct answer is option A.

Note: Morula further develops into a blastula. Morula slowly goes down towards the uterus and corona radiata get detached. Blastula from a cavity inside it and this cavity are called a blastocyst. Fertilization occurs in the fallopian tube, while implantation of the embryo occurs in the uterus.