76 J. r. HILL. 



just as happeus in tlie corresponding stage of Uasyurus (cf. 

 fig. 51 with fig. 31, though the latter represents a somewhat 

 older stage in Dasyurus). The blastocyst-wall here appears 

 relatively more extensive than in the o2-celled stage of 

 Dasyurus, an apparent difference which may perhaps be ac- 

 counted for by the difference in size of the respective eggs 

 (•24 mm. as compared with '36 mm.) . The blastomeres situated 

 adjacent to the opening and those on the right side of the 

 tigure tend to be more flattened and of greater super- 

 ficial extent than the remainder, but I can recognise no 

 difference in the cytological characters of the cells. The 

 space or cleavage cavity enclosed by the blastomeres is partly 

 occupied by a granular coagulum, and towards the opening 

 there is present a lightly stainiug reticular mass, which 

 recalls the yolk-body of Dasyurus, though I am not prepared 

 to affirm that it is of that significance. The fixation of the 

 specimen is not quite perfect. 



My next stage of Perameles is constituted by a blastocyst 

 of P. nasuta, for which I am again indebted to Mr. Moreau 

 measuring in the preserved condition '29 x "26 mm. Fig. 52, 

 PI. 3, shows a section of this blastocyst. Structurally, 

 it corresponds in all essential respects with the "43 mm. 

 blastocyst of Dasyurus, figured on the same plate (fig". 33) . 

 The blastocyst wall is complete and uuilaminar throughout. 

 It is distinguishable into two regions, a more extensive region 

 over which the cells are large and flattened and a less extensive, 

 composed of smaller but thicker cells (left side of fig. 52), 

 In the early blastocysts of Dasyurus, it may be recalled, the 

 evidence showed that the region of more flattened cells is 

 formative in significance, that of more bulky cells, non-forma- 

 tive. It is possible the same holds good for this Perameles 

 blastocyst. On the other hand, the structural condition of 

 the stage next to be described rather supports the view that 

 the smaller region, composed of {)lumper cells, is in this case 

 formative. That view seems to me the more probable of the 

 two, but there is a considerable difference in size between the 

 present blastocyst and those next available, so that it is 



