366 



I have elsewhere ^) employed the term o m p h a 1 o p 1 e u r e , signifying 

 thereby "the whole of the wall of the blastodermic vesicle or primi- 

 tive yolk-sac, beyond the region of extension of the splanchnocoele". 



In Dasyurus the larger half of this outer wall is formed by 

 the two-layered portion of the omphalopleure — the bilaminar 

 omphalopleure — (text-fig. hiJ. omph.), consisting of ectoderm and 

 entoderm only, while the remainder of its extent is constituted in 

 about equal proportions by the discoidal area of true chorion (eh.), 

 and by the relatively small annular zone of vascular (trilaminar) 

 omphalopleure {vase, omph.), coextensive with the vascular area. 



So far as concerns the general arrangement of the membranes 

 their most striking peculiarity lies in the fact that the proamnion 

 (proa.), instead of being a transitory structure as in the other hitherto 

 described Australian forms, remains persistent as in Didelphys 

 (Selenka) 2) and invests the anterior end of the embryo as far back 

 as the fore-limbs. The anterior end of the embryo, thus enclosed in 

 the proamnion, projects down into the cavity of the yolk-sac (y. c.) 

 while its posterior end, enveloped by the trunk amnion (amn.), projects 

 into the cavity of the splanchnocoele (coe.). 



The splanchnocoele is limited all round by the invaginated yolk- 

 sac splanchnopleure («/. spl), and is closed in above by the discoidal 

 area of true chorion (cli). 



The vascular area possesses the usual vessels, the sinus termi- 

 nalis (s. t.) being of relatively small size. 



The bilaminar omphalopleure {hil. ompJi.) is divisible, as is in- 

 dicated in the the text-figure, into two regions, a lower occupying 

 the lower pole of the vesicle over which the ectoderm is thin and 

 an upper forming an annular zone between the former and the sinus 

 terminalis, covered in early stages with a layer of large cubical ecto- 

 dermal cells (indicated in the text-figure by a wavy line). It is over 

 this latter area that the omphalopleural wall enters into complex 

 union with the uterine epithelium. 



As OsBORN^) and Selenka^) have recorded for Didelphys, 

 so in Dasyurus the unattached omphalopleural walls of adjacent 

 embryos fuse where they come in contact, to form common partitions 



1) Hill, The Placentation of Perameles (Contributions to the 

 Embryology of the Marsupialia, I). Q. J. M. S., Vol. 40, p. 396. 



2) loc. cit. 



3) OsBORN, The Foetal Membranes of Marsupials. Journ. ofMorphoL, 

 Vol. 1, 1888. 



4) Selenka, loc. cit. 



