Pre-Embryo - Human Development
About thirty hours after fertilization the zygote goes through mitosis giving rise to 2 blastomeres. These blastomeres go through further cleavages and by the 3rd day after fertilization, the tiny mass of cells reaches the uterus. Through this time it is a solid ball of cells called morula. In the next three days the morula is changed into a hollow ball of cells called blastocyst. The cavity of blastocyst is filled along with fluid secreted by the cells. One portion of the blastocyst consists of a concentrated mass of cells that will give rise to the entire body of the new individual. This is termed as the inner cell mass (ICM). Interestingly, each cell of this mass has the capability to develop into a complete individual. Identical twins are formed if the ICM splits. The outer ring of cells of the blastocyst surrounding its cavity is termed as trophoblast and gives rise to all the extra-embryonic membranes including much more of the placenta. This stage of development is important as the cells begin to move for the first time and become rearranged to shape the new individual. This procedure you might recall is the beginning of morphogenesis. However the major morphogenetic events occur during the third to eighth week.