140 



PEOF. OWEN ON THE HOMOLOGY 



nent oral entry thereto. In the more highly organized cartilaginous 

 fishes (Elasmobranchs) the haemal permanent mouth, or " trito- 

 stome," is also due to involution of the epiblast, forming a sac, 

 beneath the base of the brain, the closed end of the sac coming 

 into contact with the fore end of the alimentary cavity, developing 

 upward the infundibulum. Mr. Balfour sees the rudiment of the 

 hypophysis in a process of the mouth-involution which becomes 

 " constricted off." But he recognizes that the blind anterior end of 

 the alimentary canal — which he terms "throat " — is in close con- 

 tact with the "pituitary involution." This "involution becomes 

 longer and dilated terminally, while the passage connecting it with 

 the mouth becomes narrower and narrower, and is finally reduced 

 to a solid cord, which in its turn disappears. The remaining 

 vesicle then becomes divided into lobes, and connects itself closely 

 with the infundibulum." {Op. cit. p. 190.) 



In higher Vertebrates the deuto- or pseudo-pharynx (figs. 1, 2, 

 4, 8), extending to the parts ultimately modified as a pituitary 

 body or hypophysis with its onward and neurad extensions 

 — the infundibulum, third ventricle, and pineal production — 

 constitutes therewith the modified canal which traverses the 

 interspace between the homologues o£ the Invertebrate " hsem- 

 oesophageal" (fig. 3, e) and " neuroesophageal " (fig. 3, 3) brain- 

 j^asses — in Yertebrates the fore brain and following brain-parts. 

 In other words, from the neural side of the embryonal or primary 

 buccal cavity a communication (figs. 1-4, 7, s) is more or less 

 carried on toward the surface from the part where what is a 

 diverticulum from the primitive closed cesophagus (fig. 5, s') 

 seems to be seeking, as it were, its outlet at the neural aspect of 

 the body above a wide interspace (fig. 4, 5) now separating the 

 rudiment of the fore brain (e) from those of the mid (4) and 

 hind (3) brains. 



In all Invertebrates with appreciable homologues of these divi- 

 sions of the Vertebrate brain, the neural mouth (fig. 3, 7) is opened 

 at this part, the primordial attempt to attain it in the Verte- 

 brates is fulfilled, and the communication of such neural mouth 

 with the alimentary canal is completed and becomes the per- 

 sistent gullet {ih. 7, 10). 



The proposition, therefore, which I now submit to the Society 

 is, that the conario-hypophysial tract in Vertebrates is the mo- 

 dified homologue of the mouth and gullet of Invertebrates. That 

 the neur- or suboesophageal ganglion, or ganglionic masses, or 

 neural cords (fig. 3, 3, 5), constituting the centres whence are 



