THE EARLY DEVELOPMENT OF THE MAESUPLALIA. // 



impossible to decide this poiut with certainty. The blasto- 

 cyst cavity is partly occupied by coagulum. Tliere are no 

 cells present in it, bat the question of the presence of a yolk- 

 body must remain open. The shell-membrane ('0045 mm. in 

 thickness) and zona are in close apposition. 



Following this early blastocyst, I have three vesicles of 

 P. nasuta, two of them measuring* 1*8 mm. in diameter, 

 the other 1*1 mm. In their stage of development they 

 agree pretty closely with the 4'b-b mm. vesicles of Dasyurns, 

 referred to in the preceding pages under the designation 

 6, ^04, the entoderm being in process of differentiation. The 

 formative region was readily distinguishable in the intact 

 vesicles as a darker patch occupying about three eighths of 

 the surface extent of the wall. In section (PI. 8, figs. 80, 81) 

 it is characterised by its greater thickness as compared Avith 

 the non-formative or trophoblastic region, and by the 

 presence below it of numbers of primitive entodermal cells. 

 Compared with the corresponding stage in Dasyurns, the 

 chief difference consists in the relatively nnich greater thick- 

 ness of the cells of the formative region in the Perameles 

 vesicle. The latter cells are here already more or less defi- 

 nitely cubical in shape, their thickness varying from '09 

 mm. to '015 mm., and altogether they form a layer of a much 

 more uniformly thickened character than that of the 6, ^04 

 vesicles of Dasyurus. The trophoblastic ectoderm (figs. 80, 

 81, tr. ecf.) is composed of somewhat flattened cells, varying 

 in thickness from '005 to "008 mm. 



The primitive entodermal cells (figs. 80, 81, ent.) are 

 present below the formative region in fair abundance, more 

 especially around the periphery of the same, which may thus 

 appear somewhat thickened (fig. 81). The cells vary in size 

 from "01 X "007 mm. to •024 x "009 mm., and they stain on the 

 Avhole somewhat more deeply than the formative cells, to 

 whose under-surface they are closely applied. They occur 

 singly and in groups. Mitotic figures are frequently met 

 with in the cells of the formative area (observe the obliquely 

 disposed figure in one of the formative cells in fig. 81), and 



