Fertilization takes place when a free-swimming sperm cell penetrates the ovum’s protective outer layer. Once this has happened, no other sperm has an effect on the fertilized egg, or zygote.
Prior to their encounter, the egg and sperm undergo meiosis—cell division that leaves each cell with only half the usual number of chromosomes (hereditary structures) characteristic of the species. Soon after fertilization, however, the egg and sperm nuclei merge to form a zygote nucleus with the proper number of chromosomes. At the same time cleavage, a series of cell divisions (mitoses), begins.
Cleavage produces a rapidly increasing number of cells smaller than the zygote. The patterns of cleavage vary, in part because of variations in the amount and distribution of yolk. In reptiles and birds, which have a great deal of yolk in the egg, cleavage leads to the formation of a plate of cells on top of the yolk. In amphibians and mammals, with smaller amounts of yolk, cleavage results in a hollow ball, the blastula. As the cells continue to multiply, the various regions of the plate or the ball fold and shift in position. In this manner three distinct layers develop. These are known as the primary germ layers.
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