The Structure of the Foetal Yolk-sac Splanchnopleur of the Rabbit

ABSTRACT At the 16th day post coitum, a remnant of the bilaminar omphalopleur is present in the form of a short tag. The further shortening of this tag is effected by two concurrent processes ; firstly the degeneration at its free end, and secondly the pulling out of the ectodermal and endodermal epithelia of which it is composed. This latter process is accompanied by the further growth of mesoderm in the vicinity of the tag, and leads to the appearance of squamous cells in the endodermal epithelium of the yolk-sac splanchnopleur. The active production of multinucleate spherules by the chorionic ectoderm is decreasing in intensity by the 24th day. At this stage the vitello-allantoidean vessels in the mesoderm of the chorion, linking the omphaloidean and placental circulations, have mostly disappeared, only a few small vessels remaining. The apical concentration of the eosinophilic granules and mitochondria in the cytoplasm of the endodermal cells of the yolk-sac splanchnopleur, and the frequent basal position of the nucleus, suggests a secretory polarity. The manner in which the endodermal epithelium is folded and pleated suggests an absorptive function. On the side next to the exocoel the yolk-sac splanchnopleur is lined by a coelomic mesothelium of flattened cells which form a continuous pavement epithelium. The entry of circulating maternal antibodies into the foetal circulation is discussed.