DEVELOPMENT OF THE OPOSSUM 43 



In Dasyurus, on the contrary, blastocysts of 89, 93, 121, 128 

 and 130 cells are still incomplete, the last being on the point 

 of closing at the poles. The difference between the two forms 

 is due to differences existing between the respective 16-celled 

 stages. In the Australian species the two rings of cells, equa- 

 torially placed, must fill the space from the equator to the poles 

 by sheer force of crowding, as they multiply and spread. As 

 they proliferate, the cells are held in the form of a sphere by the 

 enveloping shell. In the opossum egg the cells are already dis- 

 tributed over the sphere in the 16-celled stage; hence with one 

 more division and a little flattening of the cells the blastocyst 

 wall is completed. At this stage the diameter of the eggs of 

 Didelphys and Dasyurus is about the same, 0.45 mm. But this 

 measures practically the diameter of the Dasyurine blastocyst, 

 the cells of which are flattened out under the shell of the egg. 

 In the opossum the thick albumen layer, hardly yet attacked 

 by the metabolic activity of the cells, intervenes between the 

 small blastocyst (0.12-0.15 mm. in diameter) and the shell 

 membrane (G and H, text fig. 4). 



2) First appearance of polarity. Selenka describes and figures 

 two early unilaminar blastocysts of 42 and 68 cells respectively. 

 Both exhibit an obvious polarity in that they consist of large, 

 yolk-laden cells at the lower pole, and these taper off gradually 

 to small, somewhat elongated, flatter and smaller cells at the 

 upper pole. An inspection of figure 5 discloses undoubted evi- 

 dence of polarity in the specimens of my collection. A polar 

 differentiation is then apparent for the first time in the history 

 of the egg. So far as I have been able to discover, however, the 

 difference between the cells at the two poles of the blastocysts 

 is one of thickness only, there being no difference in yolk content 

 or staining qualities of the cells. 



The differences in thickness of cells is sometimes considerable 

 (text fig. 5 E-H), sometimes hardly noticeable (text fig. 5 D) 

 and in some cases the wall appears practically uniform through- 

 out (text fig. 5 C). The shape of the section, whether circular 

 or elliptical, is not correlated with appearance of polarity. Orien- 

 tation was not feasible in imbedding the specimens, since the 



