37 FISHES CHAP. 



organs. Usually they become so displaced that the left one lies 

 in front of the right, and they appear as if median. The subse- 

 quent fate of the vesicles differs greatly in different Craniates. 

 Both persist in the Lamprey, the right vesicle to some extent 

 retaining its primitive visual function as a parietal eye and 

 directly overlying the left or pineal vesicle. In Elasmobranchs the 

 two unite to form a glandular organ, the so-called pineal body of 

 the adult, and in Teleosts the left vesicle disappears, leaving the 

 right as a pineal body. 1 There is also an embryonic median 

 outgrowth from the roof of the prosencephalon, the paraphysis, 

 which soon disappears and whose significance is not known. A 

 median hollow downgrowth from the floor of the thalamencephalon 

 forms the infundibulum, which becomes attached to a caecal 

 diverticulum from the roof of the mouth. With rare exceptions 

 the diverticulum loses all connexion with the mouth, and, as the 

 pituitary body or hypophysis, it appears as an appendage to the 

 extremity of the infundibulum. In the Crossopterygii the con- 

 nexion is retained even in the adult by means of a slender canal 

 extending from the pituitary body and opening into the oral 

 cavity. Laterally, the base of the infundibulum grows out into 

 a pair of rounded lobes, the lobi inferiores, and distally into a 

 thin-walled glandular sac, the saccus vasculosus, which lies just 

 behind the pituitary body. The cavity of the thalamencephalon 

 persists as the third ventricle or diacoele. The parts, of the brain 

 developed from the mid-brain and the hind-brain are much less 

 complicated, and, except for variations in size, they present a 

 fairly uniform character in most Fishes. 



In the mid-brain the roof bulges out into a pair of optic 

 lobes, and by the growth of lateral thickenings in its floor two 

 thick strands of longitudinally disposed nerve fibres, the crura 

 cere~bri, are formed. The cavity of the mid-brain remains as the 

 mesocoele, and from it an extension may be prolonged into each 

 optic lobe. 



From the hind-brain are formed the cerebellum or epen- 

 cephalon and the medulla oblongata or metencephalon, the former 

 as a dorsal bulging, the latter as a ventral thickening. Except 

 where the cerebellum is developed the ' dorsal wall remains 

 epithelial, and forms the roof of the persistent cavity of the 



1 In Lizards either of the two vesicles may become a parietal eye (Dendy, 

 Quart. Journ. Micr. Sci. xlii. 1899, p. 111). 



