EARLY STAGES OP DEVELOPMENT OP THE BABBIT. 147 



exhibited. The outer layer in that region is very much 

 attenuated, and shows very little sign of activity. The inner 

 layer consists all this time of rounded or ellipsoidal cells, and, 

 like the outer layer of epiblast of that region, shows very little 

 if any sign of activity. 



So little activity of division does there seem to be in the 

 inner layer of epiblast, that there is a distinct tendency for the 

 several cells to become slightly separated as in fig. 30, which 

 gives rise to the very irregular and speckled appearance of the 

 embryonic disc of the one hundred and twentieth hour. 



Very probably this is caused by the slight stretching of this 

 region. It is more noticeable at the edges than towards the 

 centre. 



"Whether there is any palingenetic meaning in this double- 

 layered condition of the epiblast I have discussed in another 

 paper. For the present, I think the right view to take of the 

 condition is that derived from the study of the actual way in 

 which the separation has originated, and to regard it as a con- 

 sequence of ontogenetic circumstances only. 



The Outer Layer of Epiblast. — This is by far the most 

 active of the embryonic layers of the fifth day. It is in an 

 active condition of growth during the whole of the day, and 

 thereby allows of the expansion of the vesicle. The character 

 of the cells seems to be just as it was during the latter part 

 of the fifth day. 



The Inner Layer of Epiblast. — This layer seems to be 

 a region of rest throughout the whole of the sixth day. There 

 is very little sign of cell multiplication. The cells are more 

 or less circular in outline when viewed from above, and 

 oval when seen laterally. They are rather scattered, and 

 thus give rise to the speckled appearance noted above. They 

 seem to be clearly separated from the outer epiblast above 

 them and from the hypoblast below them, and since they 

 are either quite separated from each other, or connected 

 only by fine strands of protoplasm at certain minute spots, 

 they are simply pulled apart by the expansion of the blasto- 

 dermic vesicle, and are not individually stretched and flattened, 



