DEVELOPMENT OF THE ELASMOBRANCH ELSHES. 83 



origin would become lost ; while if, on the other hand, it is 

 primitively a hypohlastic structure, we see from higher verte- 

 brates 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, sue i 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 modifi- 

 cations 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 la;'er, but in the course of development became budded off 

 from that layer and entered another layer, should not by a series 

 of small steps cease ever to be attached to the original layer, 

 but from the first moment it can be distinguished should be 

 found as a separate mass in the second layer. 



The changj of layers will, of course, 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 conse- 

 quences must follow. 



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



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