HOMOLOGY. 



15 



" pituitary -infundibular tract" below and the "pineal " tract 

 above. The primal fore end of the alimentary canal, ay 

 has shrunk, receded, and the communication with the 

 branchial pharynx and subsequent mouth is imminent*. 



In Ascidians a pore is formed at the front end of the 

 nervous tube leading into the part which eventually gives 

 rise to the ciliated sac, situated, in the adult, at the junction 

 between the mouth and the branchial sac. 



So in Am/pJiioxus a mouth, or oral passage, is formed, 

 which opens behind into the branchial or vascular expansion 

 from which the aUmentary canal is continued. This branchial 

 sac is on the under or hasmal side of the fore part of the 

 neural axis, issuing, in the lower division of Vertebrates, in 

 the perfection of a water-breathing apparatus, and manifesting 

 in the embryos of the higher half of the subkingdom unequi- 

 vocal traces of a branchial organization, as shown in fig. 4, lo. 

 But, although this organization subsides, the hasmal mouth, 

 or " tritostome," is in them retained. 



Having noted, briefly, the indications of an earlier or 

 neural mouth -way, or oesophagus, in the embryo of Verte- 

 brates next above the brainless AmpJiioxus, I may premise 

 that, with the appearance, in Invertebrates, of a brain in- 

 cluding both supra- (fig. 3, 6) and sub- {ib, 3) oesophageal 

 masses or ganglions — better termed, respectively, " haemoeso- 

 phageal " and " neuroesophageal " — the canal dividing them 

 is developed as a gullet " {ib. 10), and its outward opening 

 is established as a " mouth " (ib. 7) or " deutostome." 



In cerebral Vertebrates the beginning, or attempt so to 

 speak, of a canal or tubular extension directed brainward 

 is contemporaneous with the enlargements of the fore end 



* Reference may be made to the original ' Memoire ' for the explanation 

 of other letters in the above cuts. 



