DEVELOPMENT OF THE OPOSSUM 5 



maintain their radial arrangement, that is, they do not shift as 

 is the case in the Eutherian egg. With these two stages before 

 him the author could not avoid the conclusion that the third 

 cleavage is horizontal, and in view of polar differentiation of his 

 next, or 42-celled stage, he expresses the opinion that the third 

 cleavage is qualitative, dividing the four blastomeres into an 

 upper ring of ectodermal and a lower ring of entodermal cells. 



The young 'gastrulae,' as Selenka calls the 42- and 68-cell 

 stages, are unilaminar blastocysts in which a polarity is to be 

 noted, in that the cells at one pole are larger and more yolk- 

 laden than those of the other pole. The larger cells Selenka 

 considered entodermal, i.e., hneal descendants of the lower ring 

 of the 8-celled stage just described. In each case there was 

 found within the blastocyst cavity a large included cell, which 

 the author called the 'Urentodermzelle.' Hence the implication 

 is that the entoderm is formed from the lower half of the blasto- 

 cyst and from cells included within the cavity. Such an inter- 

 pretation will be shown in this paper to be incorrect, as was 

 already suggested by Hill in his study on the egg of Dasyurus. 



The next blastocysts described by Selenka evidently suffered 

 greatly in preparation and were drawn rather diagramatically ; 

 hence I am unable to make any helpful comparisons between 

 them and my own specimens. No light is thrown upon the 

 manner of entoderm formation by these two specimens. 



The other four blastocysts figured by Selenka are older, as 

 indicated by the progress in the absorption of the albumen. 

 The wall of the youngest of these is didermic for two-thirds of 

 its surface area. In the oldest of the four, which is 1.45 mm. 

 in diameter, the mesoderm and the primitive streak are already 

 laid down. The six blastocysts intermediate between these two 

 are fully formed didermic specimens and were faithfully described 

 by Selenka. It is these blastocysts with the entoderm almost 

 or quite completed of which Hill says: ''Of Selenka's early mate- 

 rial, I think it is these blastocysts alone which had any chance 

 of giving origin to normal embryos."^ It is my judgment that 



-Hill ('10), p. 11. 



