170 BTCHARD ASSHETON. 



mammals. For instance in the opossum, according to Selenka, 

 it is a single layer. But it also is not always double in the 

 Amphibia. In Triton it is at one time only a single layer of one 

 cell in thickness. Why there should be this early differentia- 

 tion into spongioblastic and neuroblastic elements in one and 

 not in another so comparatively closely allied animals it is not 

 easy to guess. Possibly it may be that, since the spongio- 

 blastic elements of the nervous system are the first to show 

 activity of growth in the nervous system (His, and above), then 

 in those animals in which, owing to various individual causes, 

 the epiblast is many-layered, the outermost layer of " cells " or 

 centres of activity being, by reason of its external position 

 more favorable to processes of respiration and so in a con- 

 dition more favorable to active growth, it will be this layer of 

 epiblast that will take on itself the earliest phase in the further 

 development of the nervous system. 



Although I think the rabbit's condition can be quite well 

 explained without reference to the above, on the other hand 

 there may be some deeper meaning in it, for which reason I 

 have thought it best to make notice of the condition in the 

 Anura. 



To make this parallelism complete and certain it is necessary 

 to show that in the rabbit the cells derived from the outer 

 layer give rise to the spongioblastic tissue, and those derived 

 from the large inner layer cells to the neuroblastic tissue. 

 This I cannot do. After a while the cells which have origi- 

 nated in each of the layers become equally active, but I have 

 as yet been unable to trace their fate respectively. 



