A cytological investigation of androgcnctic hybridsAscidia malaca(♀) ×Phallusia mamillala♂ has demonstrated that all of them contain only chromosomes ofPhallusia, in the haploid number. The embryos which developed to swimming tadpoles (11 out of 218 eggs) showed normal differentiation and organization. On the other hand, the embryos whose development stopped at the blastula stage (140 out of 218 eggs) showed neither morphogenetic movements nor differentiation, and revealed an almost complete blocking of their mitotic activity.