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 
Fic. 24.—The U-shaped tube from which the alimentary canal 
and central nervous system of vertebrates arise. N, 
nervous tube; V, intestine; N?*, neurenteric passage ; 
X, notochord; Y, yolk sac; P?, 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 
