THE ORIGIN OF THE METAZOA

Alternating phases punctuate the life cycle of protozoans. The vegetat ive phase consists of a succession of mitoses. The sexual phase covert s the vegetative cells into male and female gametes which eventually f use into a zygote. Protozoan cells successively perform the vegetative program and the sexual program. In metazoans these programs are carri ed out by different cells. Somatic cells fulfill the vegetative functi ons, while germ cells fulfill the reproductive functions. The simplest form of embryonic development implies that the zygote gives rise to j ust two kinds of cells. The article proposes a series of evolutionary transitions to explain the conversion of protozoans into metazoans and the emergence of a rudimentary embryogenesis. Colonies represent plau sible intermediates between unicellular and multicellular organisms. T hey do not undergo a proper embryogenesis as long as they are mere agg regates of identical cells. Embryonic development started when a regio n of the zygotes's cytoplasm acquired factors able to trigger the exec ution of the sexual program. Among embryonic cells, those receiving th e germinal factors took over the reproductive function. The other cell s were invested with the vegetative functions.