A B S T R A C TThe sexual differentiation in t h e colonial ascidian Botryllus schlosseri was considered with respect to colony sexualization and sexualization of t h e single zooids of t h e same blastogenic generation.The colony sexualization was investigated histologically in a series of young colonies reared in t h e laboratory and fixed periodically throughout a sequence of blastogenic generations. The newly set colonies lack gonad blastemata; next, germ cells appear and several generations are involved in various phases of hyposexuality prior to the appearance of generations in which mature both eggs and sperm.The sexual contribution of t h e zooids of the same generation was found to vary according to a n intrinsic bilateral asymmetry (the gonadogenic potential differs both contralaterally in each zooid and between zooids of a different, dextral or sinistral, parental origin) and in proportion to t h e quantity of germ cells that have been transferred from preceding generations.The transfer of germ cells from one to t h e following generations was investigated by fusing colonies of opposite pigment genotypes, then crossing t h e separated parabionts to a common genotype, and scoring t h e offspring pigment phenotypes. This experiment has shown t h a t not only immature oocytes but also undifferentiated germ cells were exchanged between t h e parabionts and then gradually matured as eggs or sperm in a prolonged series of generations.The primordial germ cells differ in size from hemoblasts, t h e source of hemocytes. However, further experiments a r e needed in order to decide between t h e two contrasting hypotheses of germ cell origin: either segregation from t h e atrial epithelium of buds or development from hemoblasts.The sexual differentiation in t h e colonies of Botryllus has been recently re-considered by Mukai and Watanabe ('76a,b). Nevertheless, many points still remain controversial.The hermaphroditic colonies reach sexual maturity only after having gone through several blastogenic generations. Male sexual maturity is attained first, though immature eggs are also present. Next, maturing gonads of either sex are produced in sequential generations. Seasonal sexual cycles in the wild, and succeeding cycles in t h e laboratory under constant temperature conditions have been described for B. schlosseri (Sabbadin, '55a, '60). Whether germ cells are present in the newly set colonies or during t h e periods or sexual inactivity and transferred to and gradually matured in t h e following generations

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