The sex ratio in litters of mice classified by the total amount of prenatal mortality.

Abstract

From studies of the normal sex ratio of living young and of abortions and still-births in man and the rat, King 1 and others have concluded that the male fœtus is less viable than the female. If this is true for the entire period of gestation there should be a negative correlation between the amount of prenatal mortality and the sex ratio. From counts of the corpora lutea of pregnancy of about 20 mice (sectioned material) Parkes 2 concludes that this is the case.

Data showing the percent of prenatal mortality for 445 litters of mice have been obtained by subtracting the number of young born from the number of ova as indicated by the number of corpora lutea. In the last week of pregnancy the corpora lutea corresponding to the litter in utero are strikingly differentiated as large hyperemic bodies protruding from the surface of the otherwise pale colored ovary. These corpora were counted in the living animals under a low power binocular microscope by means of an operation which has been shown 3 to have no influence upon the fœtuses in utero or upon the subsequent reproduction. Table I gives the total number of males and females in litters classified, according to the per cent of prenatal mortality, into five classes, each 20 per cent in width. The sex ratio shows no tendency to decline as the percentage of prenatal mortality increases; the highest ratio is found in the next to highest prenatal mortality class. The small numbers in the fifth class indicate that its low sex ratio is probably not significant. These results indicate that when the total prenatal mortality is revealed there is found no selective elimination of males.