SPINAL CORD AND MEDULLA OF CYCLOSTOMES 39 



in Polistotrema and Petronwzon show conclusively that the 

 main factor involved is the pronounced growth of the notochord. 

 It was further established that the embryonic spinal cord was 

 not only in a very plastic condition, but that the general environ- 

 ment was decidedh' favorable for bringing about a depression 

 of the spinal cord through this agency. 



lOj The conclusions outlined in (9) were cousideral)ly strength- 

 ened by the fact that a direct relationship was established, in 

 both the medulla and tail region, between the size of the noto- 

 (^hord and the amount of depression exhibited in the spinal cord. 

 This was shown in late embryos in both Polistotroina and Potro- 

 myzon, and in adult Polistotrema. 



11) That a similar depression did not take place in the higher 

 vertebrates from a growing notochord was explained by the 

 fact that the notochord is relatively a transitory and insignificant 

 structure; while in the Cyclostomes it is not only formed earl>- 

 in embryonic life, but grows rapidly and continuously. 



