Cells flatten and compact on the inside of the cavity while the zona pellucida remains in same size with the appearance of the cavity in the centre. The entire structure is now called a blastocyst (in case of human beings) or blastula or blastodermic vesicle. As a blastocyst is formed, zona pellucida becomes thinner and finally disappears. By this time the developing embryo reaches the uterus and implants into the uterine lining.
Blastocysts have three parts – trophoectoderm (trophoblast), inner cell mass and blastocoel.
Trophoblast is formed by the outer cells of morula. These cells (morula cells) begin to enlarge, flatten and converted into trophoblast by absorbing the nourishment being secreted by endometrium.
It is the outermost wall of blastocyst which does not take part in the formation of embryonic body but it forms protective, trophic membranes. These membranes eventually develop into foetal portion (chorion and amnion) of placenta. Trophoblast later becomes two layered, outer syncytiotrophoblast and inner cytotrophoblast. Cells of syncytiotrophoblast secrete hCG. The secretion of hCG begins the day the trophoblast is embedded in the endometrial lining.
It is an internal cluster of cells, which forms the body of the embryo proper.
The blastodermic vesicle, as the developing embryo gets implanted in the inner wall of the mother’s uterus with the help of the placenta. Trophoblast cells which are in close contact with inner cell mass are called cells of Rauber. Blastocoel is fluid filled cavity which helps in rapid expansion of blastocyst or blastodermic vesicle.
The blastula with blastocoel is called coeloblastula, e.
g. frog. In certain animals, the blastula is solid without blastocoel is termed stereoblastula, e.g. Neries. The blastula formed as a result of superficial cleavage is called superficial blastula, e.g. insects.
Discoblastula is formed as a result of discoidal cleavage. This blastula has many layered disc of blastomeres above the yolk, e.g. birds.