48 J. P. HILL. 



Ijlastomeres into two determinate cell-groups, respectively- 

 formative and non-formative in significance, entirely compar- 

 able with, and, indeed, even more distinct than that which 

 occurs during- cleavage in the Entheria ? I venture to think 

 that the evidence brought forward in this paper conclusively 

 justifies an answer in the affirmative to that question. 



If we assume that the upper cell-ring of the 16-celled stage 

 in Dasyurus is formative in destiny and the lower cell-ring 

 non-formative, then we might naturally expect to find in the 

 unilaminar wall of the later blastocyst some differentiation 

 indicative of its origin from two distinct cell-groups, and 

 indicative at the same time of the future embryonal and 

 extra-embryonal regions. Now just such a differentiation 

 does, as a matter of fact, become evident in vesicles 3*5 to 

 4*5 mm. in diameter. We have already seen that the wall in 

 early blastocysts "4 to "8 mm. in diameter exhibits a well- 

 marked polar differentiation in correspondence with its mode 

 of origin from the differentiated cell-rings of the 16-celled 

 stage, its upper hemisphere or thereabouts consisting of 

 smaller cells, poor in deutoplasm, its remainder of larger 

 cells, rich in deutoplasm. In later blastocysts, 1—3 mm. or 

 more in diameter, it is no longer possible to recognise this 

 distinction — at all events I have failed to observe it — but if 

 we pass to blastocysts 4*5 mm. in diameter, in which the wall 

 is still unilaminar, we find on careful examination of the 

 entire vesicle under a low power that there is now present a 

 definite continuous line, which encircles the vesicle in the 

 equatorial region so as to divide its wall into two hemi- 

 spherical ai*eas (PI. 4, fig. 38, /.L). If we remove and stain 

 a portion of the wall of such a vesicle, including this line, 

 and examine it microscopically (figs. 42-46), it becomes 

 apparent at once, from the disposition of the cells on either 

 side of the line, that Ave have to do with a sutural line or line 

 of junction produced by the meeting of two sets of cells, 

 which are pursuing their own independent courses of growth 

 and division. The cells never cross the demarcation line 

 from the one side to the other, but remain strictly confined 



