370 FISHES CHAP. 
organs. Usually they become so displaced that the left one hes 
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.t 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 saecws 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 erwra 
cerebri, 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 
1JIn Lizards either of the two vesicles may become a parietal eye (Dendy, 
Quart. Journ. Mier. Sci. xiii. 1899, p. 111). 
