THE EARLY DEVELOPMEXT OF THE MARSUPIALIA. 65 



rentiation iu different vesicles of this stage, and even in 

 different parts of the formative region of one and the same 

 vesicle. In some vesicles there are relatively few primitive 

 entodermal cells, in others they are much more abundant. 

 Fig. 68, from the formative region of an ^04 vesicle, shows a 

 typical patch of them and illustrates very well the highest 

 stage of differentiation which they attain in these vesicles. The 

 entodermal cells therein depicted all appear to be definite! v 

 internal, and it is especially worthy of note that the ])ortion 

 of the unilarainar wall in relation to them is composed exclu- 

 sively of the larger, lighter staining cells. It is these cells 

 which directly form the embryonal ectoderm .of the blastocysts 

 next to be described. The entodermal cells are obviously 

 amoeboid in character (observe especially the cells near the 

 middle of the figure), and are in active process of linking 

 themselves together into a cellular reticulum. In fio*. 69 is 

 shown a small portion of the formative region of another '04 

 vesicle. A single entodermal mother-cell in process of 

 division occurs in position in the unihiminar wall, which is 

 otherwise composed of ectodermal cells, whilst internally there 

 are present three entodermal cells, already linked together by 

 their pseudopodial processes. The two lowermost cells affui-d 

 especially striking examples of amoeboid activity, the elongated 

 pseudopodial process of the cell on the left terminating in a 

 well-marked reticulation in definite continuity with the corre- 

 sponding, but shorter and thicker process of the cell on the 

 right. 



3. Establishment of the Definitive Embryonal 



A r e a . 



Following directly on the stage represented by the '04 

 blastocysts described in the preceding section is one desig- 

 nated in my list as 5, 18 . vii . 01 and referred to here as 5, '01. 

 It comprises twenty-two blastocysts obtained from a female 

 killed fifteen days after coition and all normal, with the 

 exception of one Avhich was shrivelled, and all in precisely 



VOL. 56, PART 1. NEW SERIES. 5 



