DISUSE AND ITS EFFECTS. 



49 



which is of large relative size in the embryo and lined 

 by epithelium. The significance of this canal has long 

 puzzled anatomists. A new interest attached to it 

 when Kowalevski discovered in the embryo of ascidians 

 and amphioxus, that this central canal is directly con- 

 tinuous with the intestine. This temporary connection 



FIG. 24. The U-shaped tube from which the alimentary canal 

 and central nervous system of vertebrates arise. N, 

 nervous tube ; V, intestine ; N 1 , neurenteric passage ; 

 X, notochord ; Y, yolk sac ; P 1 , pineal ; and P. Pituitary 

 diverticulum. 



has been observed in all the great groups of vertebrata. 

 even in the human embryo. 



In 1887 I was able to furnish evidence that this central 

 canal of the cord, and a portion of it prolonged into the 

 brain, may be regarded as originally a segment of intes- 

 tine which has become disused for alimentary purposes 



5 



