SEGMENTATION OF THE OVUM OF THE SHEEP. 243 



In making that statement I was confident that Robinson 

 was not right in the manner of application of his suggestions 

 to the rabbit. His remarks on the blastocyst of the rabbit 

 will be found on pp. 420 — 422 (46), part of which I must 

 quote here. " It is stated that the flat cells on the outer 

 surface of the inner mass either disappear entirely, or they 

 fuse with the cells immediately beneath them to form the 

 epiblast of the germ ; and in either case, after the flat cells 

 over the outer surface of the inner mass have disappeared, it is 

 just as possible that the remainder of the vesicle wall hangs 

 in continuity with the peripheral flattened cells derived from 

 the inner layer of the inner mass — that is, with the hypoblast, 

 as with those which constitute the epiblast; but in the case of 

 the rabbit there is no evidence which will completely substan- 

 tiate a statement that either the one or the other of those 

 possibilities occurs. It is certain that the portion of the 

 vesicle wall, which is at first formed by a single layer of 

 flattened cells eventually becomes didermic, and that the 

 change from the single to the double layered condition com- 

 mences in the vicinity of the inner mass, whence it gradually 

 extends to the opposite pole of the ovum. It is stated that 

 the didermic condition is produced by the extension of the 

 hypoblast round the inner surface of the primitive wall, but 

 no satisfactory proof has been brought forward in support of 

 this statement. The cells of the extending layer are from 

 the first flattened, like those over which they are extending, 

 and nuclear division has not been clearly demonstrated either 

 in the inner or the outer layer." 



" It appears to me, therefore, that the evidence which has 

 been obtained from the study of the rabbit's ovum does not 

 conclusively substantiate the statement that the outer wall of 

 the primitive blastocyst is epiblastic in nature, and that the 

 hypoblast extends round its inner surface.^' 



I paid much attention to this, and I can have no doubt that the 

 rabbit's blastocyst is not capable of receiving the interpretation 

 that Robinson suggests. The cells, which apparently " extend 

 round ^' the blastocyst, are beyond all doubt on the inside. 



