The first stage of development, during which the male and female gametes fuse, is the precondition for the formation of a new human being and is called fertilization (stage 1)
The fusion of the oocyte II and the spermatozoon triggers the completion of the second meiotic division in the oocyte. The oocyte and spermatozoon nuclei (each containing 23 chromosomes) unite in the course of the fusion of their genetic material and thus form the zygote (46 chromosomes).
The fertilized human oocyte wanders from the ampulla through the fallopian tube to the uterus in order to implant itself there on about the 6th day (stage 4). After 4 days the young embryo consists of roughly 16 to 32 cells (blastomeres) and resembles a mulberry. One terms this the . The separation of the blastomeres into embryoblast and trophoblast is accomplished in this stage.
In the course of its further migration it divides (cleavage) and forms hundreds of daughter cells without increasing its volume. Each of these daughter cells (blastomere) has the same set of chromosomes as the zygote and thus contains the entire genetic information.
These morula cells serve as the origin for the embryo and its membranes, and also for the placenta and its associated structures. The development and differentiation continues with the cleavages. After the eight-cell stage the outer, originally round blastomeres, which stick to each other only a little, begin to flatten out. The contact among them is strengthened by special connections (tight junctions). This compaction (consolidation) leads to a partitioning of the cells, whereby one part goes to the center and the other forms the periphery.
The embryo arises from the inner cells (thus also called the embryoblast).
The outer cells form the first source for the placental membranes and have been given the name trophoblast. On the 4th day of development (stage 3) the morula, which now consists of roughly 30 cells, begins with assimilating fluid (that is initially secreted by the blastomeres and fills the intercellular space). Under the influence of the hydrostatic pressure of this fluid, a hollow space arises in the morula, the so-called blastocyst cavity.
The cells of the embryoblast (inner cells) form a compact assembly on one side of the cavity and the outer cells - or trophoblast – organize themselves into an epithelial layer. In this stage the embryo is called a. The side on which the inner cells are located becomes the embryonic pole; the opposite side is called the abembryonic pole.