Question

2.
How is a zygote formed? ​

Answer 1

Answer:

fusion of male gamete and female gamete forms a zygote

Answer 2

After the egg is fertilized, the male and female pronuclei join to form a true nucleus. Now we have a zygote at the single-cell stage. About 24 hours later, the cell begins mitosis, forming two identical cells. Each of these cells is called a blastomere. The cells continue to divide for a couple of days until they form a solid ball of cells, called a morula. About day 4, the ball of cells (which are still dividing) begins to hollow out, becoming a blastocyst. The cells of the blastocyst continue dividing, and around this time the blastocyst reaches the uterus and attaches to the uterine wall. The layer of cells on one side of the blastocyst is thicker- this is the embryoblast, which will eventually become the embryo. The rest of the hollow ball of cells (the trophoblast) is still only one cell thick, but will start to become thicker as the blastocyst becomes embedded in the uterine wall. The trophoblast has cells that will become extramniotic membranes (the amnion, etc) and cells that will become the placenta.

By the third week the zygote is encapsulated in the uterine wall and the placenta is beginning to form- it is not a zygote anymore, it is an embryo.