78 J. P. T[ILT-. 



they also occur in the primitive entodermal cells. Examina- 

 tion of the sections leaves no donbt in one's mind as to the 

 source of the entodermal cells. They are undoubtedly derived 

 from the formative region of the vesicle wall. The shell- 

 membrane has a thickness of about "0027 mm. 



2. Macropus. 



Of Macropus the earliest stnge I have examined is a blasto- 

 cyst of M. ruficoUis, •25 x '21 mm. in diameter. It is not 

 in a quite perfect state of preservation, but is in a sufficiently 

 good condition to enable me to say that the wall is complete 

 and unilaminar throughout, just as in the '29 x "26 mm. 

 blastocyst of Perameles. The shell-membrane has a thickness 

 of about 'OO'D mm., and there are still remains of the albumen 

 between it and the zona. 



My next stage (figs. 82-85) is a blastocyst of the same 

 species, ■35 mm. in diameter. It unfortunately suffered in 

 preparation, but practically the whole of the formative area 

 of the blastocyst wall and part of the trophoblastic ectoderm 

 are comprised in the sections (PI. 9, fig. 82), so that it is still 

 possible to make out its chief structural features. In its stage 

 of development this blastocyst closely agrees with the last 

 described blastocysts of Perameles. The formative ai-ea of 

 the wall is perfectly distinct in the sections because of its 

 greater thickness and the presence below it of the primitive 

 entodermal cells. It attains its greatest thickness ("027 mm.) 

 peripherally, whilst it is thinnest centrally ("006 mm.), so that, 

 taken as a whole, it is not quite such a uniformly thickened 

 layer as is that of the Perameles blastocysts. Primitive ento- 

 dermal cells are present below it, but not in great abundance 

 (figs. 82, 84, 85, ent.). In fig. 88, a formative cell is seen in 

 division, the axis of the spindle being oblique to the surface. 

 The trophoblastic ectoderm (figs. 82, 83, tr. ect.) is composed 

 of the usual flattened cells, and varies in thickness from 

 •005 to -0007 mm. 



In the blastocyst cavity, adjacent to the trophoblastic 



