12 



CONARIO-HYPOPHYSIAL TRACT. 



diminishes and becomes excluded from the abdomen by com- 

 pletion of the walls of that cavity, save where the primitive 

 yolk-canal, fig. 4, 19, passes on to the shrunk vitellicle, now 

 shut out as an appendage, ultimately to be absorbed or cast 

 off at birth. Here, therefore, besides the transitory vent, we 

 have a primordial " mouth " and " gullet," or parts holding 

 functionally, for a brief period, those relations to the digestive 

 sac. The persistent indication of such course of the em- 

 bryonal food is called " umbilicus :" it points to one inlet of 

 food which has made way for another ; and that other will 

 make way for a third. As well devote pains and speculation 

 to the " function " of the navel as to analogous renmants of a 

 later communication with the alimentary canal, doomed like- 

 wise to obliteration with concomitant solidification of parts. 



In low radiate forms of Hfe, Medusa e. g., the vitelline 

 entry, or " protostome," is permanent ; a " deutostome " may, 

 in like manner, appear as another step in the rising scale 

 which is not parted with. 



But to return to our Vertebrate grade. The alimentary 

 tube, parallel with the myelonal one, communicates or anas- 

 tomoses therewith; a common canal thus results, but of 

 which the hsemal portion will be modified to give sustenance 

 to the body, the neural portion to the mind. In the course 

 of differentiation the caudal intercommunion is abolished, the 

 "blastopore" being partially annexed by the neural canal. 

 The anterior end of the alimentary tube (fig. 4, ii), extending 

 forward, comes into close contact and continuity with the 

 canal, which may be described as commencing below at 

 the part which becomes the " pituitary " and its continuation 

 the " infundibulum ;" thence continuing upward (neurad) by 

 the third ventricle to the base or origin of the pineal pro- 

 duction of the thalamencephalon, which production, per- 

 forating, as in the embryo Iguana, the soft lamellar basis of 



