24 CARL GOTTFRIED HARTMAN 



the yolk, but the future surface of the blastomere is distinctly 

 outlined by a marginal, yolk-free zone along which the mem- 

 brane of the blastomere will form. In other words, one blasto- 

 mere was caught in the act of eliminating its yolk. In the other 

 specimen, No. 46 (1) shown in figure 10, both blastomeres have 

 eliminated the yolk, but one has rounded off in advance of the 

 other. After the cells have separated and become fully formed, 

 the yolk remaining within them comes to have the same dis- 

 tribution in a submarginal region as in the unsegmented egg. 

 This peripheral distribution is to be seen in blastomeres up to 

 the 16-celled stage (fig. 23). The eliminated yolk and cyto- 

 plasmic substance, together with the layer of albumen, is later 

 completely but slowly absorbed. Considerable yolk, in varia- 

 ble amounts, is retained even within the cells which form the 

 walls of the younger blastocysts. But this yolk is also gradually 

 absorbed until in late unilaminar blastocysts the fat granules 

 are only occasionally observed. 



It follows, then, from the absence of polar distribution of yolk 

 in the opossum egg as compared with the egg of Dasyurus, that 

 the yolk of the former is not given off as in the latter. In the 

 opossum egg the yolk, being peripherally distributed, is given 

 off peripherally; being concentrated in a dense mass at one 

 pole in the egg of Dasyurus, the yolk is, in this case cast out at 

 the pole when the two blastomeres form. The egg of the opossum 

 is also much smaller (text fig. 4) and probably also proportionately 

 less laden with yolk, for its egg measures on the average about 

 one-half the diameter, that is, one-eighth the volume, of the egg 

 of Dasyurus. 



THE 4-CELLED STAGE 



a. The orientation of the blastomeres 



1) Shifting of the blastomeres. The orientation of the blasto- 

 meres in the 4-celled stage presents the chief point of divergence 

 in the cleavage of Didelphys and Dasyurus, the former follow- 

 ing the manner of Eutheria. The conclusions here presented 

 are based upon abundant material secured from females just 

 captured, as well as from cage animals. 



