OEIGIN OP METAMEBIO SEGMENTATION. 77 



development of a living form. S al en sky ^ long ago pointed 

 this out. With a slight change in the shape of the anterior 

 end of the body of the Ascidian larva in Kowalewsky's figure, 

 the mouth would be removed from what we call the dorsal 

 (neural) to what we call the ventral (abneural) surface. This 

 would involve a flexure of the anterior end of the neural canal, 

 and, I think, gives a clue to the phylogenetic meaning of the 

 cranial flexure. The closure of the anterior pore of the neural 

 canal is efi'ected in such a way that it leaves a trace on the one 

 hand as the infundibulum ; on the other as the pituitary body. 

 This homology has been often suggested. The persistence of 

 the lower part of this pore, and its development from the 

 epiblast of the buccal cavity, may be explained by supposing 

 that the buccal end of the pore was glandular before the closure 

 of the neural canal was efi'ected. When this closure was 

 effected, the buccal part remained in connection with the 

 mouth as an excretory organ, a state of things persisting, 

 according to Salensky, in Ascidians. It then acquired the new 

 functions which it has at present, lost in the adult its connec- 

 tion with the mouth, and is known to us as the pituitary body. 

 Meanwhile, some of the endoderm cells of the dorsal wall of 

 the alimentary canal have become specially modified and 

 separated from the rest as the notochord. 



(2) In the Vertebrata the anterior end of the notochord is 

 bent round, and becomes connected with the pituitary body at 

 its extreme front end.2 This condition of the anterior end of 

 the notochord may be seen in the embryo before the pituitary 

 involution is cut off from the ectoderm of the developing 

 mouth — that is to say, the relation of the anterior end of the 

 notochord to the ectoderm is similar to that of the hind end ; 



' Salensky, 'Zeitschrift f. "Wiss. Zoologie,' vol. xxvii, p. 212; and 

 'Morpliol. Jahrbuch./ vol. iii, p. 600. 



^ The relation of the anterior end of the notochord to the pituitary body is 

 somewhat complicated. For the knowledge of this fact I am iadebted to Mr. 

 Heape, who is at present engaged in investigating this very point. He in- 

 forms me that the existence of the connection was known to the older embryo- 

 logists (W. Miiller). 



