Foetal and placental size in a Peromyscus species

and their reciprocal hybrids were collected. Foetal weight and crown\p=m-\ rump length were ascertained, as were placental weight and decidual surface area. No significant differences in these measurements were found when the parent species were compared, but the reciprocal hybrids dif-fered significantly from each other and from the parent types in means for these characters. Placentae from reciprocal hybrids showed more than a five-fold difference in weight. They also differed markedly in histo-logical structure. Foetal weight was positively correlated with placen-tal weight in all crosses. These findings are discussed with reference to immunological factors in placentation