44 J. P. HILL. 



form a complete lining to the blastocyst, wliicli thereby 

 becomes bilamiuar. Such a marked enlargement of the blasto- 

 cyst prior to the differentiation of the embryonal ectoderm 

 and entoderm as is here described for Dasyurus does not 

 apparently occur, so far as known, in other Marsupials : in 

 Perameles, for example, the embryonal ectoderm and the 

 entoderm are in process of differentiation in vesicles a little 

 over 1 mm. in diameter (v. p. 77), in Macropus these two layers 

 are already fully established in a vesicle only "8 mm, in 

 diameter (v. p. 79), and much the same holds good for Tricho- 

 surus and Petrogale. It is paralleled by the marked growth 

 which in the Monotremes follows the completion of the blasto- 

 cyst and which precedes the appearance of embryonal diifer- 

 entiation. It must be remembered, however, that the groAving 

 blastocyst in the Monotreme is bilaminar and not unilaminar 

 as in Dasyurus, owing to the fact that the entoderm is estab- 

 lished as a complete layer at a very much earlier period than 

 is the case in the latter. I am nevertheless inclined to regard 

 the attainment by the Dasyurus blastocyst of a large size, 

 prior to the differentiation of the embryonal ectoderm and the 

 entoderm, as a more primitive condition than that found in 

 other Marsupials. The pronounced hypertrophy which the 

 uteri of Dasyurus undergo during the early stages of gesta- 

 tion, an hypertrophy which appears to be proportionately 

 greater than that met Avith in other forms,^ is no doubt to be 

 correlated with the presence in them of such a considerable 

 number of actively growing blastocysts. 



Selenka states (Heft 5, p. 180) that he examined seven 

 blastocysts of Dasyurus " -j^ mm." in diameter, taken from a 

 female fifteen days after copulation. He describes their 

 structure as follows : " Man unterscheidet (1) eine sehr 

 zarte aussere, homogene Haut (Granulosamembrau), (2) 



' For example, the uteri of a female (5, 18 . vii . 'Ol) from wliicli I 

 obtained twenty-one normal vesicles, 4'5-6 mm. in diameter, with the 

 embryonal area definitely estaldished, measured as follows : Left uterus, 

 4'5 X 4'7 X 1'4 cm. (fourteen vesicles) ; right uterus, 4'5 x 4"2 x 1'45 cm. 

 (seven vesicles and one shrivelled). 



