THE EARLY DEVELOPMENT OF THE MARSUPIAL [A. 35 



marked polarity. The nucleus lies excentrically in the 

 upper half of the cell, just above the equator, and is sur- 

 rounded by a finely granular zone of cytoplasm, outside which 

 is a thin irregular ring of deutoplasmic material. The cyto- 

 plasm of the apical part of the cell is clear and relatively free 

 from deutoplasm ; that of the lower half, on the other hand, 

 is so rich in deutoplasm as to appear quite dense and opaque. 

 The conclusion is therefore justified that the blastomeres of 

 the 8-celled stage possess a definite polarity^ which has been 

 acquired as the result of the progressive concentration of 

 deutoplasmic material at their vegetative poles during the 

 cleavage process. Division, in the equatorial plane, of cells 

 so constituted must necessarily be unequal and qualitative, so 

 far at least as the cytoplasm is concerned. 



In the 13-celled stage three of the original eight blasto- 

 meres are in process of division, and five have already divided 

 unequally and qualitatively, so that 3 + (5 x 2) = 13, and in 

 the 14-celled stage two of the original blastomeres are in 

 division and six have already divided : 2 + (G x 2) = 14. 



The significance to be attached to this characteristic unequal 

 and qualitative division of the blastomeres of the 8-celled stage 

 to form two superimposed cell-rings, markedly diffei'entiated 

 from each other, we shall presently consider. Meantime I 

 may categorically state the conclusions I have reached in 

 regard thereto. The wall of the blastocyst in Dasyurus is at 

 its first origin, and for some considerable time thereafter, 

 unilaminar throughout its entire extent, and I regard the 

 upper cell-ring of the 16-celled stage as giving origin to 

 the formative or embryonal region of the unilaminar wall, 

 the lower cell-ring as furnishing the extra-embryonal or non- 

 formative remainder of the same. I shall therefore refer to 

 the upper cell-ring and its derivatives as formative or 

 embryonal, and to the lower cell-ring and its derivatives 

 as non-formative or extra-embryonal. 



The formative or embryonal region furnishes the embryonal 

 ectoderm and the entire entoderm of the vesicle, and I accord- 

 ingly conclude that it is the homologue of the embryonal knot 



