Fertilization and Development

The egg cell finishes meiosis after spermatozoon’s penetration. Fusion of gamete nucleuses leads to origination of zygote that starts mitotic divisions.

At the beginning cells are undifferentiated. Both cells that origin in the first division can transform into a human being. If fertilized egg splits at the early stage of development identical twins will result. Not identical twins originate from two fertilized ova.		morula - 32 cell stadium of zygote
	identical twins

As the time passes cells start differentiating. In human body there are more than 200 types of cells. They all take rise from the fertilized egg cell. The structure and functions of the cell are determined by expression of a certain set of genes. It is assumed that development is controlled by arranged (in time an space) expression of genes. One gene’s product activate another gene. This way embryo goes through succeeding stages of development. Some regulators are sensitive for the type of nearby cells. During divisions the genetic decisions about functions of the future cells are made. The daughter cells take over the limitations of the parent cell. This leads to origination of specialized cells.

Studies of embryogenesis are advanced for fly Drosophila melanogaster and round-worm Caenorhabditis elegans. The organisms on which mammal embryogenesis is investigated are mice.