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.