The outer layer of cells of morula absorbs fluid which collects in a cavity formed within the cell mass.
This cavity is called as blastocoel or blastocyst cavity. The outer layer of Hat cells are called as trophoblast. The embryo is now called as blastocyst or blastula. It is composed of an outer layer of trophoblast and inner cell mass.
The trophoblast surrounds blastocoel and the inner cell mass. The inner cell mass looks like a small knob at one pole and it gives rise to the embryo.
The trophoblast does not take part in the formation of the embryo proper.
It gives rise to the extra-embryonic membranes, such as, chorion and amnion which give protection and collect nourishment for the embryo. As the blastocyst is formed, zona pellucida becomes thinner and finally disappears. (Fig. 3(B). 11)
Implantation of Blastocyst (Fig. 3(B). 12):
Implantation is the attachment of blastocyst to the wall of the uterus of the mother. It occurs about week after fertilization. The portion of the blastocyst where the inner cell mass is located, lies against endometrium (= inner most layer of the uterus). At the point of contact the trophoblastic cells digest and engulf the endometrial cells and thus, blastocyst sinks into a pit formed in the endometrium.
Then the blastocyst is completely buried in the endometrium. The blastocyst forms villi by means of which it establishes intimate contact with the uterine wall. The blastocyst derive nutrition and lives as a parasite within the uterus of the mother. Chorion is an extra embryonic membrane which develops from the trophoblast. It secretes a hormone called human chorionic gonadotrophin which maintains corpus luteum and stimulates it to secrete progesterone.