THE EARLY DEVELOPMENT OP THE MAESUPL\LIA. 41 



I have met with one specimen, an incomplete blastocyst 

 •39 mm. in diameter (belonging to the same batch as the 

 other blastocysts referred to in this section^), in which the 

 lower hemisphere would appear to have been completed before 

 the upper, for the yolk-body lies in contact with the zona in 

 the region where the cellular wall is as yet absent, and that 

 the yolk-body has not been secondarily displaced is proved by 

 a micro-photograph of the specimen in my possession (taken 

 immediately after its transference to the fixing solution), in 

 which the yolk-body is seen to lie at the unclosed pole in 

 exactly the same position as in the sections. 



In connection with this exceptional specimen, it may be 

 recalled that Selenka, in his 68-celled " gastrula " of Didelphys 

 (fig. 10, Taf. xvii), figures the wall as complete at the lower 

 pole, the '^ blastopore " having already closed, but as still in- 

 complete at the upper pole, there being present a small opening 

 leading into the blastocyst cavity. In the 42-celled ''gasti'ula" 

 (fig. 8, Taf. xvii) this same opening and the "blastopore" as 

 well are present. The occurrence of these openings at 

 opposite poles, and the general agreement in the constitution 

 of the blastocyst wall (larger, more yolk-rich cells at lower 

 pole, smaller, less yolk-rich cells at upper), in the corre- 

 sponding stages in Didelphys and Dasyurus justify the con- 

 clusion that the blastocyst of the former develops in the same 

 way as does that of the latter. It is worthy of remark, 

 however, that the just completed blastocyst of Didelphys 

 appears to be considerably smaller than that of Dasyurus. 

 Selenka unfortunately gives no measurements of his early 

 stages, but I have calculated from the figure, the magnification 

 of which is given, that the 68-celled blastocyst has a diameter 

 of about -137 mm. The corresponding stage of Dasyurus 

 measures about "39 mm., and is therefore nearly three times as 

 large. 



' This batch, fvoni female 2 B. 16 . vii . '01, comprised altogether 

 twenty-eight eggs, of which some eighteen were normal complete and 

 incomplete blastocysts (■39--49 mm. in diameter) and ten abnormal, four 

 of these being unsegmented ova. 



