Answer: Option A
:
A
The single-celled zygote divides by mitosis, and this division has a special name - it is called cleavage, forming 2 cells. The first division occurs about a day after fertilization, and subsequent divisions occur every 12 to 24 hours after that. From here on, the collection of cells wouldbe broadly called an embryo until the 8th week of gestation. During this period, the embryo further divides to form 4 cells,which divide to form 8 and then 16 cells, and so on. These daughter cells that are formed are known as blastomeres. Once the embryo has reached 8 to 16 blastomeres, it is known as a morula.Soon after the number of cells of the morula increases and they start to “differentiate”, the trophoblast cells secrete fluid into the center of the morula to create a fluid-filled space inside, called the blastocoel. Now that the embryo has taken the form of a hollow ball of cells, it is called a blastula.