Abstract
A combination of mouse inbred strains was studied, in which foetal but not placental weight is greater in the F1 hybrid than in the inbred litters. Gestation period was found to be shorter for the F1 hybrid than for the inbred litters. It is concluded that, insofar as the length of gestation is inversely related to the mass of the conceptuses, it is the foetus rather than the placenta which matters.