FCETAL MEMBRANES OF OPOSSUM AND OTHER MARSUPIALS. 481 



in the older opossum embryos, while the allantois was a small 

 sac supplied by two arteries. 



The line of attachment of the yolk-sac to the subzonal mem- 

 brane was marked, as described by Professor Chapman, by the 

 sinus terminalis, and this, as in the opussum, seemed to give 

 rise to a single vein near the edge of the disc ; there was a 

 single artery. The foetus therefore closely resembled the earlier 

 opossums, and it was very gratifying to discover minute 

 villi all over the attached area of the yolk-sac, thus 

 confirming the previous observations. These villi were so 

 minute that it is not at all surprising that they were overlooked 

 by previous observers. I have not yet had an opportunity 

 of examining them closely ; their external size and appearance 

 was similar to those found in Specimen 2, although they were 

 in an early stage of development. Beneath them the disc 

 formed by the yolk resembled closely that of the opossum, and 

 it was quite evident that at a later period they would resemble 

 in internal structure those found in Specimen 2. 



I think we may now regard the following facts, in respect 

 to the foetal membranes of the Marsupials as fairly well 

 established. 



1. That the yolk-sac at an early stage spreads over the inner 

 surface of the subzonal membrane, forming a disc-like chorion, 

 which in the kangaroo and opossum is bounded by the sinus 

 terminalis. This chorion may become extensive in the later 

 stages. The subzonal epithelium then gives rise to hollow 

 conical upgrowths of columnar cells. From the epithelium of 

 the yolk-sac there arise papillae, which become vascular, while 

 the subzonal cells become very much flattened. The rudi- 

 mentary villi thus formed, in the early oposssum and kangaroo 

 embryos, are thickly distributed over the area surrounded by 

 the sinus terminalis, but in other forms they may extend beyond 

 this area. 



2. The allantois arises in the same way as in the Placentalia 

 at quite an early stage of development, and soon becomes vas- 

 cular. In the kangaroo, if it unites with the subzonal mem- 

 brane at all, it is only in the later period of gestation. In the 



