408 JAS. P. HILL. 



{d) Vascular Omphalopleure and Yolk-sac Pla- 

 centa. — The vascular omphalopleure includes, as already 

 pointed out, the area of the yolk-sac wall between the peri- 

 phery of the true chorion and the sinus terminalis, and is 

 co-extensive with the vascular area. Its entoderm (figs. 6 and 

 7, vase, omph.) consists of a fairly thick, somewhat flattened 

 layer of varying width, with oval or rounded nuclei staining 

 deeply like the protoplasm. The unsplit mesoderm is a thin 

 and delicate layer carrying the very numerous capillaries of 

 the vascular area. 



Tiie ectoderm is especially noteworthy on account of its 

 extreme tenuity (fig. 6, ect.). It consists of a delicate thin 

 layer of mostly flattened cells with small oval or fusiform 

 nuclei. Here and there at intervals, cells of a somewhat 

 triangular shape are found, with their outer pointed ends 

 projecting beyond the level of the general surface of the 

 ectoderm. 



In Didelphys, according to the descriptions both of Osborn 

 and Selenka, the ectoderm of the vascular omphalopleure is a 

 much thicker layer than in Perameles. Osborji describes it 

 as consisting of '* elongated cells with amoebiform processes 

 which are closely applied to the lining epithelium of the 

 uterus" (11, p. 378 ; cf. also his fig. 4, PI. xvii) ; and Selenka, 

 describing the layer in a 7| days' embryo, says (2, p. 137), 

 " Bei weitem der grosste Theil dieser Ektodermzellen dehnt 

 sich aus und nimmt blasige Form an, unter gleichzeitiger 

 Vergrosserung der Kerne (Taf. xxviii, fig. 5, d.). In diesem 

 Felde von blasigen Zellen bemerkt man gegen Ende der Incu- 

 bation vereinzelte Fleckchen kleiner nahezu kubischer Ekto- 

 dermzellen (c und a) und hier und da sah ich an Schnitten 

 sogar Andeutung von Zottenbildung {b) mit axialen Meso- 

 dermzellen." Again, in a 5 — 6 days' old Hypsiprymnus he 

 describes the ectoderm of the vascular omphalopleure in the 

 following words (2, p. 184) : — '^ Doch zeigten sich die Ektoderm- 

 zellen des Chorion auff'allend vergrossert und gegen die 

 Uterushohle Zapfen-oder kuppelartig vorgewolbt (Tafel xxxii, 

 Figur 2)." Caldwell, again, referring presumably to Phasco- 



