THE EARLY DEVELOPME>fT OF THE ^EARSUPIALIA. 67 



a small portion of the Hue shows with sufficient distinctness, I 

 think, to demonstrate its identity witli that of the precedino- 

 stage. 



The vesicle wall in all my sections of this stage appears 

 to be somewhat thinner than that of the '01 blastocysts, hat 

 apart from this apparently variational difference the present 

 blastocysts are almost exactly intermediate between the latter 

 and those next to be described. 



The embryonal ectoderm (fig. 78, emh. ect.) appears in 

 section fairly uniformly thickened, though its cells are still of 

 the flattened type. In surface view in in to to preparations 

 (cf . fig. 48), they exhibit the same polyg'onal form and lightly 

 staining qualities as the larger cells of the formative region 

 of the '04 blastocysts, which we have already identifi^ed as 

 prospective embryonal ectodermal cells. The jnnctioual line 

 between the embryonal ectoderm and the extra-embryonal is 

 now for the first time readily distinguishable in sections 

 (fig. 78). The extra-embryonal ectoderm (tropho-ectoderm) 

 (PL 5, figs. 48 and 49, PI. 8, fig. 78, tr. ect.) differs in no 

 essential respect from the corresponding layer in the '04 

 blastocysts. 



The entoderm in these blastocysts is exceedingly closely 

 adherent to the inner surface of the embryonal ectoderm and 

 cannot be removed therefrom by artificial means. It varies 

 slightly in its character in different vesicles and in different 

 parts of its extent in the same vesicle. Mostly it appears as 

 a continuous thin cell-layer (figs, 49 and 78, ent.), but here and 

 there patches occur in which the cells form a reticulum quite 

 similar to that shown in fig. 6S of the preceding stage. 



The next stage (designated in my list as 8 . vi . 01), and the 

 last of Dasyurus that need be described in the present com- 

 munication, comprises eleven vesicles (5-5*5 mm. in diameter), 

 in Avhicli the embryonal area is conspicuous and distinctly in 

 advance of that of the preceding vesicles, but is still devoid 

 of any trace of embryonal differentiation (PI. 4, fig. 40; 

 PI. 8, fig. 79). 



The embryonal area is hemispherical in form (its greatest 



