244 RICHARD ASSHETON. 



Hubrecht (35) says in reference to Tupaia javanica, in 

 speaking of E-obinsou's " ingenious speculations/' " According 

 to these views^ the outer layer of the monodermic blastocyst 

 is in reality a hypoblastic layer. The didermic phase essen- 

 tially originates out of this by a gradual spreading of epiblast 

 cells outside the more primitive hypoblastic wall. However 

 ingenious these speculations may be, the author holds them to 

 be erroneous." He then proceeds to give an account of the 

 development of Tupaia javanica ''as an example of a 

 mammal, the early development of which furnishes us with 

 decisive evidence in this respect." 



So, although Robinson's conclusions cannot be maintained 

 for the rabbit when applied in the way he did, there seems to 

 me to be no great objection to placing an interpretation upon 

 a much earlier stage, in accordance with my discoveries in the 

 sheep, as explained above. 



I would, therefore, suggest a modification of Minot's and 

 Robinson's theories of a hypoblastic blastocyst as follows : 



1. The whole of the subzonal epithelium is entodermic. 



2. The central portion of the inner mass of the mammalian 

 blastocyst is ectodermic, but the surrounding layer is ento- 

 dermic. 



3. *' The cavity of the mammalian blastocyst does not 

 correspond with the segmentation cavity of the lower Verte- 

 brates, but with the archenteron" (Robinson). 



4. The archenteron is at first a closed vesicle ; there is no 

 blastopore. 



In criticising Robinson's suggestions with regard to the 

 rabbit, it must be remembered that he has described a stage in 

 the development of the ferret (47), in which the blastodermic 

 vesicle of the eleventh day is said to be a thin-walled vesicle of 

 hypoblast, bearing at one pole and on the outside, i. e. between 

 the main wall of the vesicle and the zona radiata, a small disc 

 of cells — the epiblast. 



Thus Robinson finds in the ferret what he expected to be 

 the case in the rabbit. 



If the blastocyst of the ferret becomes, as we may suppose, 



