To date, it has been impossible to fertilize the oocytes of ripe Pecten maximus obtained by gonad stripping as it is routinely done in hatchery with Crassostrea gigas . The events occurring between artificial induction of spawning and normal fertilization were studied experimentally in order to explain this difference. Fertilization and rearing trials were performed on oocytes obtained from the gonads of both species by scarification, by aspiration from genital ducts (of P. maximus only) and after spawning. Every batch of oocytes was fixed for histological study. Serial sections of the gonad of a fixed spawning P. maximus were made to reveal modifications of the nucleus of the oocytes during their expulsion. In P. maximus the stripped oocytes can be fertilized and are viable only when spawning is initiated. Histological sections show that germinal vesicle breakdown occurs at this very moment. The same was observed in C. gigas but it appears that in this species sperm can also induce the oocyte maturation of stripped oocytes.