Fertilisation and embryonic development

Ontogenesis is the individual development of an organism (the term was introduced in 1866 by E. Haeckel); it is divided into embryonic and postembryonic periods. The embryonic period lasts from the zygote to birth or hatching: cleavage, gastrulation and organogenesis. In cleavage the zygote divides rapidly by successive mitoses (2, 4, 8, 16, ... blastomeres); the cells do not grow, so they get smaller; the result is a one-layered blastula with a fluid-filled cavity (blastocoel). In gastrulation the two-layered gastrula forms: outer ectoderm and inner endoderm; in most animals mesoderm appears between them. From ectoderm come the skin epithelium, nervous system and sense organs; from endoderm the intestinal epithelium, liver, pancreas and lungs; from mesoderm the muscles, skeleton, blood, heart and the excretory and sex organs. In organogenesis the neural tube, notochord and gut tube appear (neurula). In embryonic induction one part of the embryo directs the development of another (H. Spemann, 1920s).