After the fertilization, the zygote undergoes a series of rapid divisions called cleavage. The zygote first divides into two cells known as blastomeres (30 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 12-16 cells, the morula. These repeated mitotic divisions happen during the zygote's passage through the uterine tube toward the uterus.
Four days after fertilization, when the morula enters the uterus, fluid-filed spaces between the blastomeres appear and fuse into a central cavity called the blastocoele. At this stage of the development, the conceptus is called a blastocyst.

1. Zona pellucida
2. Blastmere

Blastocyst later loses the zona pellucida through the process called blastocyst hatching. The blastocyst consists of inner cell mass (embryoblast) and outer cell mass (trophoblast). 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 6th day after fertilization, the blastocyst attaches to the endometrial epithelium with it's embryonic pole. This triggers the differentiation of the trophoblast into an inner cytotrophoblast and an 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.