After spermocytes enter the female they are not able to fertilize the ovary right away. They must first go through a period of capacitation where uterine substances dissolve the outer proteins of the sperm cell. Then an acrosomal reaction occurs where acrosomal vesicle content is released that help the sperm to digest the membrane around the oocyte so the sperm can fuse with the oocyte. Once one sperm enters the egg cell, the membrane undergoes a process known as a cortical reaction where the membrane no longer allows any other sperm to enter. After this process, the fertilized egg or zygote undergoes cleavage where the it rapidly goes through a series of mitotic divisions where the cells get increasingly smaller. It soon becomes a ball of these smaller cells known as a morula. However the pressure of the cells on each other turns the morula into a hollow ball of cells called a blastula the cavity of which is called a blastocoel. Following the development of the blastula, cells begin to migrate into a hole in the blastula known as the blastopore. The blastula almost folds in on itself. This new folded ball of cells is called the gastrula. It contains a new cavity called the archenteron. The gastrula is composed of three layers of cells: the endoderm or the inner layer, the mesoderm or the middle layer that is composed of the migrated cells, and the ectoderm or the outer layer of cells. The three layers soon undergo organogenesis or the development of organs. The ectoderm becomes the skin, the outer layer of the eyes, and the nerves. The mesoderm becomes the organs, muscles, and skeleton. The endoderm becomes the inner lining of these organs.