THE EAELY DEVELOPMEXT OF THE MARSUPIALIA. 9 



men, judging- from Dasyurus, is normal as regards the consti- 

 tution of its wall and. the occurrence of an opening at each pole. 

 The lower opening, however, has no blastoporic significance, 

 but, like the upper, owes its presence to the mode of formation 

 of the blastocyst-wall by the spreading of the blastomeres 

 towards the poles of the sphere formed by the egg-envelopes. 

 Selenka's blastopore simply marks the last point of closure. 

 This specimen I hold to be abnormal from the presence of 

 the so-called '' urentoderm " cell in its interior. I figure 

 (PI. 3, fig. 37) a section of a fairly comparable and un- 

 doubtedly abnormal blastocyst of Dasyurus in which there is 

 also present in the blastocyst cavity a large free cell. Here 

 this latter is unquestionably a blastomere of the lower hemi- 

 sphere, which, having failed to divide, has become enclosed 

 by the spreading of its neighbours. Selenka's " ureutoderm- 

 zelle " 1 regard as a similarly displaced blastomere.] 



A 68-celled "gastrula" (figs. and 10) is next described. 

 It is essentially similar to the preceding, only the "blasto- 

 pore " has closed. 



The succeeding stage (fig. 11) is a somewhat older 

 *' gastrula," in Avhicli gastrulation is said to be still in 

 progress, since over the lower pole, in the i-egion of the now 

 closed blastopore, it is no longer possible to say which cells 

 belong to the ectoderm, which to the entoderm. The latter 

 layer is described as being several cells thick in the blastoporic 

 region, and as in course of spreading round inside the ecto- 

 dermal wall of the" gastrula" towards the upper or animal 

 pole. [This specimen is undoubtedly abnormal, at all events 

 there is no comparable stage in Dasyurus. It is difficult to 

 obtain a clear idea of Selenka's conception of the mode of origin 

 of the germ-layers, but he evidently held (cf . pp. 116 and 119) 

 that the large yolk-rich cells of the lower (" blastoporic ") 

 pole constitute the anlage of the entoderm, and that they 

 become inturned at the " blastopore " and proliferate to form 

 the definitive entoderm, which then gradually extends round to 

 the animal pole, in contact with the inner surface of the wall 

 of the gastrula, that wall forming the ectoderm. He appa- 



