764 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



into the infundibulum. The apex of the infundibulum is enlarged, 

 and to it is attached a pituitary body (pt.}. 



The mid-brain is of considerable size, and consists of a basal 

 portion connecting the optic thalami with the medulla, and a 

 pair of large optic lobes (<?/./.). The iter a tertio ad quartum 

 ventriculum, which forms the ventricle of this part of the brain, 

 is prolonged into each optic lobe, and the floor of each prolon- 

 gation is taken up by a dome-shaped projection, the homologue 

 of the torus semicircularis of Teleostei. 



The hind-brain consists of the usual parts, the medulla 

 oblongata and the cerebellum. The medulla presents no peculiar 

 features. The sides of the fourth ventricle are thickened and 

 everted, and marked with peculiar folds (Plate 38, figs. 47 A 

 and B, m.o.). 



The cerebellum is much larger than in the majority of 

 Ganoids, and resembles in all essential features the cerebellum 

 of Teleostei. In side views it has a somewhat S-shaped form, 

 from the presence of a peculiar lateral sulcus (Plate 38, fig. 47 A, 

 cb.}. As shewn by Wilder, its wall actually has in longitudinal 

 section this form of curvature, owing to its anterior part pro- 

 jecting forwards into the cavity of the iter 1 . This forward pro- 

 jection is not, however, so conspicuous as in most Teleostei. 

 The cerebellum contains a large unpaired prolongation of the 

 fourth ventricle. 



II. Development. 



The early development of the brain has already been de- 

 scribed ; and, although we do not propose to give any detailed 

 account of the later stages of its growth, we have thought it 

 worth while calling attention to certain developmental features 

 which may probably be regarded as to some extent characteristic 

 of the Ganoids. With this view we have figured (Plate 37, figs. 

 44, 45) longitudinal sections of the brain at two stages, viz.: 

 of larvae of 15 and 26 millims., and transverse sections (Plate 37, 

 figs. 46 A G) of the brain of a larva at about the latter stage 

 (25 millims.). 



1 In Wilder'* figure the walls of the cerebellum are represented as much too thin. 



