Final answer:
Immediately after an embryo forms, it starts to replicate new totipotent stem cells through a process known as cleavage. These initial divisions involve mitosis without an increase in the overall size of the embryo, eventually forming a morula and then a blastocyst.
Step-by-step explanation:
Immediately after an embryo forms, it begins to replicate new stem cells. The most likely type of cells to form in the first few divisions are totipotent stem cells. These initial embryonic cells are the product of the zygote division through a process known as cleavage.
By the second day after fertilization, the single-celled zygote undergoes mitosis to form two daughter cells. Mitosis continues at intervals of 12 to 24 hours, resulting in four, then eight, and so on until about sixteen cells by day 4, forming what is known as a morula. The early mitotic divisions are termed cleavage, which leads to more cells, but the overall volume remains the same due to confinement within the zona pellucida.
As division continues and the number of cells increases, a fluid-filled cavity develops, forming a structure known as a blastocyst. This stage eventually leads to further differentiation and the development of the organism.