HOMOLOGY. 



7 



or oral outlet, but is checked by the external skin in lower 

 forms, and by the cranial roof of the brain in the higher ones. 

 Where its course is not so arrested, the peripheral and ex- 

 panded part becomes consolidated ; and such mass has been 

 described, like the similarly modified opposite or internal 

 portion of the abortive gullet, as a gland. 



In the class of Fishes the relative magnitude and tubular 

 character of this transcerebral tract is still more marked, 

 examples of which I have elsewhere described and figured*. 



According to Prof. Ehlers, " Die Epiphyse am Gehirn der 

 Plagiostomen," the representative of the pineal body, appears 

 as an elongate membranous tube opening at one, the central, 

 end into the mid brain, and expanding at the peripheral end, 

 where the canal is arrested either by the cranial cartilage, in 

 a cavity of which it is lodged, or by the epicranial skin after 

 perforation of the cranium. 



The relative dimensions of this abortive pharynx remain 

 greatest in the still lower fishes. In the Ammocetes the 

 walls of the peripheral part of the " pineal " canal are plicate : 

 in the Lampreys the canal, arrested after traversing the mid 

 brain, retains a sac-like expansion which extends both back- 

 ward and forward beneath the skin of the head. 



The further change in this abortive pharynx is the closure 

 of its lumen, and the conversion of the tube into a cord, 

 which retains, after traversing the epicranium, its larger pro- 

 portions, and forms the mass which, in Amphibia and Eep- 

 tiha, is called " cerebral gland " {ante, p. 4) ; but this term 

 is as applicable to the pituitary as to the pineal body. 



The true vasculo-membranous infundibuliform downward 



* 'Anatomy of Vertebrates,' 8vo, vol. i. 1866, p. 277. "The third 

 ventricle in Osseous Eishos is prolonged downward into the pedicle of the 

 hypophysis or ' pituitary gland,' fig. 185, j>, and upward into that of the 

 conarium or ' pineal gland,' fig. 175, t/-. " 



