ON THE DEVELOPMENT OF THE ELASMOBRANCH FISHES. 343 



hand, it is primitively a hypoblastic structure, we see from 

 higher vetebrates how, by becoming separated from the 

 hypoblast rather earlier than in the Dog-fish, viz. at the same 

 time as the rest of the mesoblast, its primitive derivation 

 from the hypoblast has become concealed. 



The view seemingly held by many embryologists of the 

 present day, that an organ, when it was primitively derived 

 from one layer, can never be apparently formed in another 

 layer, appears to me both unreasonable on a priori grounds 

 and also unsupported by facts. 



I see no reason for doubting that the embryo in the 

 earliest periods of development is as subject to the laws of 

 natural selection as is the animal at any other period. 

 Indeed, there appear to me grounds for the thinking that it is 

 more so. The remarkable differences in allied species as to 

 the amount of food-yolk, which always entail corresponding 

 alterations in the development — the different modes of 

 segmentation in allied species, such as are found in the 

 Amphipoda and Isopoda — the suppression of many stages 

 in freshwater species, which are retained in the allied 

 marine species — are all instances of modifications due to 

 natural selection affecting the earliest stages of development. 

 If such points as these can be affected by natural selection 

 I see no reason why the arrangement of individual cells (or 

 rather primitive elements) should not also be modified ; 

 why, in fact, a mass of cells which was originally derived 

 from one layer, but in the course of development became 

 budded off from that layer and entered another layer, shou Id 

 not by a series of small steps cease ever to be attached to 

 the original layer, but from the first moment it can be dis- 

 tinguished should be found as a separate mass in the second 

 layer. 



The change of layers will, of couise, only take place 

 where some economy is effected by it. The variations in the 

 mode of development of the nervous system may probably be 

 explained in this way. 



If we admit that organs can undergo changes, as to the 

 primitive layer from which they are derived, important 

 consequences must follow. 



It will, for instance, by no means be suiRcient evidence of 

 two organs not being homologous that they are not deve- 

 loped from the same layer. It renders the task of tracing 

 out the homologies from development much more difficult 

 than if the ordinary view of the invariable correspondence of 

 the three layers throughout the animal kingdom be accepted. 

 Although I do not believe that thjs oorves])on deuce is invari- 



