Fragmentation (cleavage).

After the fertilization, the zygote undergoes a series of rapid divisions called cleavage. Human cleavage is:

- complete (cleavage is characterized by complete division of the mass of the fertilized egg to form the blastomeres);

- unequal (cleavage in which the dark and light blastomeres are unequal in size).

The zygote first divides into two cells known as blastomeres (thirty hours after fertilization). Three days after the fertilization, a rapid increase in the number of the cells results in the formation of a solid ball of twelve to six cells, the morula. These repeated mitotic divisions happen during the zygote’s passage through the utrerine tube towards the uterus. Four days after fertilization, when the morula enters the uterus, fluid-filled spaces between the blastomeres appear and fuse a central cavity called the blastocoele. At this stage of the development, the conceptus is called a blastocyst.

Blastocyst later loses the zona pellucida through the process called blastocyst hatching. The blastocyst consists of two parts, an outer layer of small slightly flattened cells (light blastomeres) the trophoblast, and an inner cell mass consisting of a small group of larger polyhedral cells (dark blastomeres) contained within the trophoblastic vesicle. The cavity of the blastocyst (blastocoele) separates the trophoblast from the inner cell mass except for the small area where they are in contact. Embryoblast gives rise to the embryo and a part of the amnion. The trophoblast cells form most of the extraembryonic membranes, i.e., the bulk of the placenta. During the sixth day after fertilization, the blastocyst attached to the endometrial epithelium with it’s embryonic pole. This triggers the differentiation of the trophoblast into an inner cytotrophoblast and outer syncytiotrophoblast. By the end of the first week, the blastocyst is superficially implanted. At about seven days a flattened layer of cuboidal cells, called hypoblast (primitive endoderm) appears on the surface of the embryoblast.

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