Studies on the sex-ratio and related phenomena.—I. Fœtal retrogression in mice

During recent years an increasing amount of evidence has been accumulated relative to the death and retrogression of embryos and fœtuses. Pre-natal mortality resulting in abortion is a phenomenon of obvious occurrence, but the internal removal of dead fœtuses and embryos is quite another matter. Most of the records which exist on this latter process relate to polytocous animals, in which the question is obviously different from monotocous animals. In multiple pregnancies the abortion of a dead fœtus could only take place at the sacrifice of the rest of the fœtuses, and such being the case a means of removing the dead fœtus without terminating the pregnancy would be a considerable advantage to the animal. In monotocous animals cases of fœtal retrogression appear to be less common, though this may be accounted for by the increased difficulties of observation. The supposition of re-absorption has been challenged on the grounds that decomposition could hardly proceed in company with gestation. It seems, however, that in some animals certain products of gestation are normally assimilated by the maternal organism. Robinson (15) says that the maternal tissues are not shed at the time of birth in the mole, and that some of the fœtal tissues are retained to be absorbed later. Jenkinson (4) also states that inPerameles the allantoic blood vessels are regularly absorbed through the agency of maternal leucocytes by the parturient uterus and that fœtal tissues are absorbed somewhat similarly inDasyurus. Also the order of the disappearance of the conceptus is suggestive. Meyer (14) says that the embryo. or fœtus is the first member of conceptuses to be removed, and my own observations show that the membranes are intact long after decomposition has started in the fœtus. This strongly suggests that the process is one of organized absorption rather than of putrefactive activity, which would presumably begin with the membranes.