THE EAELY DEVELOPMENT OF THE MAKSUPIALTA. 45 



daruiitei' ein Lagei* von Ektodermzellen, welclie im Gebiete 

 des Embryonalschildes piMsmatich, ara gegeuiiberliegenden 

 Pole iiahezu kubischj im ubrigen abgeplattet erscheinen, (3) 

 ein inneres zusammenliangendes Lager von abgeflachten Ento- 

 dermzellen." This description, apart from the reference to 

 the thin shell-membrane, is entirely inapplicable to blastocysts 

 of Dasynrus of the mentioned size which I have studied. 



I have examined a practically complete series of vesicles of 

 Dasyurus ranging from '4 mm. to 4 mm. in diameter and all of 

 them without exception are unilaminar. 



Of vesicles under 1 mm. diameter I possess serial sections 

 of more than two dozen, ranging from '5 mm. to '8 mm. in 

 diameter, and obtained from three different females. These 

 differ structurally in no essential respect from the just com- 

 pleted blastocysts. A surface view of a blastocyst "6 mm. in 

 diameter is shown in fig. 63, PI. 6; in this the difference in 

 the cytoplasmic characters of the cells of opposite hemispheres 

 is clearly brought out, the non-formative cells of the lower 

 hemisphere having much more marked perinuclear zones of 

 dense C3'toplasm (deutoplasm) than the formative cells of the 

 upper hemisphere ; moreover, the former cells tend to be of 

 larger superficial extent than the latter. Fig. 34, PI. 3, 

 represents a section of a blastocyst "57 mm. in diameter, and 

 fig. 35 a section of one '73 mm. in diameter. These blasto- 

 cysts differ in no essential way from the '43 mm. blastocyst 

 represented in fig. 33. As in the latter, the cellular wall is 

 unilaminar throughout, but both it and the shell-membt-ane 

 have undergone considerable attenuation. Moreover in these 

 blastocysts, apart from the clue afforded by the shrivelled 

 yolk-body, it is pi'actically impossible to determine from the 

 sections which is morphologically the upper hemisphere and 

 which the lower. In fig, 36, from a '6 i7im. blastocyst, on the 

 other hand, the cells of the hemisphere opposite the yolk-body 

 {y.h.) are larger than those of the hemisphere adjacent to 

 which that body is situated. In the "57 mm. blastocyst the 

 shell-membrane has a thickness of '0052 mm., in the '73 mm. 

 blastocyst it measures "0045 mm., and in a '84 mm. blastocyst 



