THE EARLY DEVELOPMENT OF THE ilARSUPIALLV. 55 



Eutlierian blastocyst and the less dense to the extra-embry- 

 onal region of the same-, but just the reverse pi'oves to hold 

 true for the '04 vesicles, the formative hemisphere in these 

 appearing less dense than the non-formative. I cannot now 

 test my foi-mer inference by direct observation since I do not 

 appear to have any of the '99 vesicles left intact, but amongst 

 my in toto preparations of the vesicle wall I find one 

 labelled as from the " lower pole " which unmistakably 

 belongs to the formative hemisphere, hence I conclude that 

 the denser and slightly smaller region which I originally 

 regarded as formative is really non-formative, a conclusion 

 which brings the '99 vesicles into agreement with the '04 

 batch. 



In these latter vesicles the satural line and the two regions 

 of the wall can be quite readily made out on careful examina- 

 tion under a low power with transmitted light. The one 

 region appears slightly denser (darker) and has more closely 

 arranged nuclei (i.e. is composed of smaller cells) than the 

 other. On the average this denser region appears to be 

 rather the less extensive of the two ; the two regions may be 

 about equal ; on the other hand the denser may be the smaller. 

 Examination of stained preparations of the wall demonstrates 

 that the darker hemisphere is non-formative, the lighter, 

 formative, it would therefore seem that in certain of these 

 '04 vesicles the formative region has grown more rapidly than 

 the non-formative. 



In stained prepai'ations of the wall both of the '99 and '04 

 vesicles, the differences between the two hemisphei-es are now 

 so well marked that there is no difficulty in referring even an 

 isolated fragment to its proper region. The non-formative 

 hemisphere differs in no essential way from that of the '01 

 vesicles, and as in these, is readily distinguishable from the 

 formative by the much greater uniformity in the size and 

 staining properties of its cells (fig. 45), as well as by the fact 

 that there are no primitive eutodermal cells such as occur in 

 relation to the formative hemisphere, in connection with it. 

 Its constituent cells are on the average distinctly smaller than 



