Sex difference in mouse embryonic development at neurulation.

Sixty-seven mouse embryos from 10 litters collected on the morning of Day 9 of gestation, when neurulation is beginning, were classified according to the precise stage of development reached, and sex-chromatin analysis was performed on the yolk sac. Within litters, the least developed embryos were more likely to be female than male, while the most advanced embryos were predominantly male. Taking all embryos, the mean somite number was greater in males than females.