Blastulas, Gastrulas and the First Animals

Abstract

A blastula consists of similar, non-specialized cells resulting from divisions of a metazoan egg. Most metazoans go through such a phase in their early development, and it is widely assumed that the blastula represents an early phase in animal evolution. I believe, however, that blastulas, like all embryos and larvae, were later additions to life-histories, not recapitulations of ancestral forms. This view opens the way to new suggestions on the origins of the Metazoa.