Question

Mammalian blastula is known as
a. Blastodisc
b. Discoblastula
c. Stereoblastula
d. Blastocyst

Answer 1

Hint: Generally after the fertilization, and formation of the zygote, the single-cell zygote begins to divide into two cells, and four cells and into 16 cell stage called morula later it forms into blastula and implants to uterine walls.

Complete answer:
- In mammals, after the zygote formation is completed, the single-cell zygote again begins to divide and cleavage of cells takes place.
- In Day 2 after the formation of the zygote, just there are 2 to 4 cells
- Even at day 3 also there is no change in the size of the zygote and it comprises 5 to 8 cells.
- Then after it reaches the 16 cell stage by cleavage of cells, which is termed as a morula.
- On Day 5, the changes begin to take place, where the 16 cell stage morula undergoes the cavitation.
- In this stage of the embryo, the differentiation of cells begins, where it consists of an outer tight layer of cells called trophoblast, in future this trophoblast converts to the placenta.
- And the embryo in this stage has inner cell mass, it is also called embryoblast, and they also contain a fluid-filled cavity called blastocoel.
- At this stage, rapid cleavage occurs and in this stage of differentiated cells with a fluid filled cavity is called blastula and in mammals, this stage is called Blastocyst.

Hence, The correct answer is option (D).

When coming to other options, Stereoblastula is the blastula seen in annelids and molluscs, and discoblastula is seen in reptiles and birds and blastodisc, these all are wrong options.

Note: Generally it takes 5 days for the zygote to reach the blastocyst stages and it reaches the phase of implantation by day 7, where this stage of embryo gets implanted to the uterine endometrial walls of the female uterus for further organogenesis.