THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 47 



mental importance, since it apparently becomes lost at an 

 early period during the g-i'owth of the blastocyst. Such an 

 assiimptiou, however, would be very wide of the mark, as I 

 hope to demonstrate in the next section of this paper, and, 

 indeed, in view of the facts already set forth, is an altogether 

 improbable one. 



Reappearance of Polar Differentiation in the 

 Blastocyst Wall. — Following on the period of what may 

 be termed the preliminary growtb of the blastocyst, in the 

 course of which the original polar differentiation in the 

 blastocyst wall apparently becomes obliterated, is an 

 extremely interesting one, during which that differentiation 

 again becomes manifest. In view of the fact (1) that the 

 fourth cleavage in Dasyurus is of the nature of a qualitative 

 cytoplasmic division, and (2) that approximately one half or 

 rather less of the unilaminar vesicle wall is formed from the 

 eight smaller and less yolk-rich cells of the upper ring of the 

 ]6-celled stage, and its remainder from the eight larger 

 more yolk-rich cells of the lower ring, it thus becomes a 

 question of the first importance to determine if we can the 

 significance of that differentiation. 



Amongst the Eutheria, it has been conclusively shown by 

 various observers (Van Beneden, Heape, Hubrecht, Assheton, 

 and others) that there occurs during cleavage an early 

 separation of the blastomeres into two more or less distinctly 

 differentiated groups, one of which eventually, by a process 

 of overgrowth, completely encloses the other. The peripheral 

 cell-group or layer forms the outer extra-embryonal layer of 

 the wall of: the later blastocyst (the trophoblast of Hubrecht, 

 or trophoblastic ectoderm as I prefer to term it). It therefore 

 takes no direct part in the formation of the embryo, and may 

 be distinguished as non-formative. The enclosed cell-group, 

 termed the inner cell-mass or embryonal knot, gives rise, on 

 the other hand, to the embrj^onal ectoderm as well as to the 

 entire entoderm of the vesicle, and may accordingly be dis- 

 tinguished as formative. May it not be, then, that we have 

 here at the fourth cleavage in Dasyurus a separation of the 



