Early embryonic development: the protostomes and deuterostomes

Abstract

A consequence of sexual reproduction in Metazoa is that a multicellular adult has to develop from a single cell. This is an extremely complicated and hazardous process involving repeated cell divisions and cell movement to permit organ development and growth. Embryonic development, that is the development of the individual within the fertilized egg, may be a short or lengthy process. A short embryonic stage, which is more usual in invertebrates, results in a very small juvenile unable at first to follow the adult mode of life. A larval period forms part of the development process, the larvae frequently looking totally unlike their parents and following a different mode of life. Alternatively the period of embryonic development may be prolonged, in which case the embryo has to be provided with a source of food (commonly yolk) and hatches as a miniature replica of the adult, adopting the same mode of life.