EAULT STAGES OF DEVELOPMENT OE THE RABBIT. 129 



embryo at the present day is a question of very great dif- 

 ficulty. 



It can hardly be looked upon as a cavity comparable to the 

 segmentation cavity of Araphioxus or Amphibia, or the Lam- 

 prey or Ganoids, for it has a fate different from that in any of 

 those animals. The fate of the segmentation cavity of the 

 above-mentioned animals is to disappear and take no part in 

 the formation of the cavities of the adult. 



The cavity of the blastodermic vesicle whose formation we 

 are about to discuss never disappears, never, at any rate, in 

 the rabbit, as part of it remains as the cavity of the alimen- 

 tary canal of the adult. Part of it undoubtedly is comparable 

 to the archenterou; possibly all of it is, for it becomes the 

 gut-cavity of the adult. 



The first cause which produces the cleft that subsequently 

 enlarges into the cavity of the blastodermic vesicle may be a 

 more active growth of the outer layer of cells. Undoubtedly 

 there is after this time a more active growth of the cells of 

 the outer layer. They increase much more rapidly than the 

 cells of the inner mass. 



It is not easy to explain why the energy which up to a 

 certain time causes a solid morula should do so no longer. 

 Why should not the morula steadily increase in size, but be 

 still a morula ? Although I believe that the subsequent vast 

 increase in size of the blastodermic vesicle cavity is due to the 

 diffusion inwards of fluid derived from the uterus, still this can 

 hardly be the cause of the first starting of a cleft as in fig. 21. 

 This seems to be best explained by the assumption of an in- 

 crease in rate of growth of the outer cells over the inner cells 

 of the morula. That such an increase does exist the following 

 table provides evidence. 



In a median section through an embryo : 



