Studies on Sterility and Prenatal Mortality in Wild Rabbits

Abstract

1. The yolk-sac contents were observed to be gelatinous in many embryos of wild rabbits at from 7- to 12-day stages, but particularly at 8 and 9 days. Gelatinous strands were observed in the uterine lumen, often connecting adjoining blastocysts, in 8-12-day stages, but particularly at 8 and 9 days. Gelatinous blastocysts and strands frequently were encountered together in the same uterus. 2. Evidence is adduced that these abnormalities, though tending to occur at slightly earlier stages, are related to the mortality which attains a maximum on the 11th and 12th days. 3. Microscopic examination revealed the presence of a reticulum of fine fibrils in the yolk-sac cavity, the histological characters of which are described. The degree of development of this reticulum varied widely from one uterus to another, and often in the individual embryos of the one litter. The presence of this reticulum, when sufficiently dense, was responsible for the gelatinous character of the yolk-sac contents. The gelatinous strands consisted of felted masses of these fibres and of maternal and foetal tissue debris. 4. The fibrous reticulum was absent from the cavities of the amnion and exocoele. 5. A similar reticulum was present sometimes, though rarely so well developed, in the embryos of tame rabbits. Frequently it was absent. 6. It was shown by histological means that the reticulum was not an artefact. The alternatives, that it was either a micro-organism or a network of organic fibres, remained. 7. Morphologically the reticulum resembled the mycelium of an actinomycete so closely that the possibility that it was an invasive organism belonging to this group could not be ignored. Culture experiments disproved this theory and provided satisfactory evidence that the yolk-sac cavities of tame rabbit embryos containing the reticulum were bacteriologically sterile. 8. Sections of clots of fibrin prepared from rabbit-blood plasma presented a similar histological appearance. 9. The presence of fibrinogen in the yolk-sac fluid of 9-day embryos of tame rabbits was demonstrated by clotting the aspirated and citrated fluid with thrombin. Comparison of the clots formed in parallel series of dilutions of the yolk-sac fluid and of plasma indicated that the concentration of fibrinogen in the former was of the order of 50% of that in the latter. 10. Separation of the fibrin from the residue of the yolk-sac fluid was effected by filtration and repeated washing. The mean weight of dried fibrin obtained per ml of yolk-sac fluid was 37% of that from plasma and its nitrogen content was 33%. The amount of fibrin in the yolk-sac fluid varied significantly from litter to litter. The mean quantity of nitrogen in the residue of the yolk-sac fluid was 57 % of that in the blood serum. 11. The significance of these results in relation to embryonic nutrition and prenatal mortality is discussed.