THE PLACENTA.TION OF PERAMELES. 417 



preceding, show conclusively that with the advent of the 

 allantoic placenta the yolk-sac circulation is giving place to 

 the allantoic. The latter, indeed, is now the predominant one ; 

 and we may add that just as, in the preceding stage, most of 

 the blood coming from the yolk-sac placenta passed directly to 

 the heart, so now most of the blood coming from the allantoic 

 placenta passes by way of the left allantoic vein and the ductus 

 venosus Arantii directly into the inferior vena cava. 



The question whether the yolk-sac placenta remains func- 

 tional, though in a diminished degree, throughout the whole 

 period of intra-uterine life of the embryo ; or whether, as seems 

 likely from comparison with other placental mammalian forms, 

 it soon after this stage gives entire place to the later appear- 

 ing allantoic placenta, can only be definitely decided when 

 further material is available. As tending to support the latter 

 alternative, it may be pointed out here that in the next (post- 

 partum) stage, while the allantois was still found adherent to 

 the syncytium of the placental area, no portion of the omphalo- 

 pleure was to be found in the uterine cavity. Further, the 

 syncytium outside the allantoic placental area no longer 

 showed a richly vascular surface, but was rapidly retrogressing, 

 and indeed was already partly covered by the regenerating 

 uterine epithelium. The syncytium of the allantoic placental 

 area, on the other hand, though in process of absorption, had 

 not altered to such a marked degree. 



These facts render it probable that the omphalopleure breaks 

 up and disappears some time before the end of intra-uterine 

 life (cf. also next section). 



Unfortunately I am unable to give any details as to the 

 relative dimensions of the vascular area in this and the pre- 

 ceding stage. It may, however, be mentioned that the vessels 

 of the vascular area in this stage are apparently not nearly so 

 richly developed as in a Macropod embryo of about the same 

 developmental stage. 



(e) Bilaminar Omphalopleure. — This presents features 

 tending to suggest that it is even now in process of degenera- 

 tion. The ectoderm has on the whole become greatly flattened 



