DEVELOPMENT OF THE OPOSSUM 49 



differentiation may be made out for the first time in the develop- 

 ment of the egg. This difference in the cells of the two poles is 

 merely quantitative, so far as may be observed from a study of 

 the specimens at this stage alone. The only evidence of polarity 

 at this stage lies in the thinning of the cells at one pole of the egg. 

 The question as to which of the poles is formative and which 

 non-formative will be discussed in the next section. 



THE FORMATION OF THE ENTODERM 



a. Entoderm formation as conjectured by Selenka 



The method of entoderm formation in the opossum natually 

 interested Selenka, but he lacked the requisite stages to deter- 

 mine the matter. It has been shown above that the included 

 cell in Selenka's 42- and 68-celled stages, celled by him 'Urento- 

 dermzelle,' has no morphological significance. His surmise, 

 moreover, that the 8-celled stage is made up of four ectodermal 

 and four entodermal cells is a mere conjecture, based solely on 

 analogy. Hence it follows also that the early blastocyst is not 

 divided into a lower thick-walled 'entodermal' half and an upper 

 'ectodermal' half, as he suggested. 



Selenka's later blastocysts (his two 'young gastrulae') already 

 contain entodermal (?) cells massed at one pole of the egg. From 

 these masses proliferation of flat cells are seen advancing along 

 the blastocyst wall. Little or no differences exist in the cells of 

 the ectodermal wall, the polarity of the egg being indicated 

 merely by the position of the entodermal cells. The ectodermal 

 cells are represented by the author rather ideally as a recon- 

 struction, so that I am at a loss to make accurate comparison 

 with my material. In my collection I find several eggs of batch 

 No. 144 (fig. 27) which show a massing of cells at one pole, 

 though not in the numbers represented by Selenka's young 

 gastrulae. On the whole I believe these two specimens of 

 Selenka to be badly shrunken. Hence, also, his attempt to 

 figure out the axis of the gastrula from the shape of one of his 

 specimens must be characterized as futile, for the differences in 

 the several diameters of the blastocyst were one of chance 

 merely. The blastocysts secured by me are mainly spherical; 



JOURNAL OF MORPHOLOGY, VOL. 27, NO. 1 



