DEVELOPMENT OF ELASMOBRANCH FISHES. 181 



teretes of human anatomy. Part of tlie root of the seventh 

 nerve originates from them. They project freely in front into 

 the cavity of the fourth ventricle (PI. XV. fig. 7 ft). 



By stage Q restiform tracts are indistinctly marked off from 

 the remainder of the brain, and are anteriorly continued into 

 the cerebellum, of which they form the peduncles. Near their 

 junction with the cerebellum they form prominent bodies (PI. 

 XV. fig. la, r t), which are regarded by Miklucho-Maclay^ as re- 

 presenting the true cerebellum. 



By stage the medulla presents posteriorly, projecting into 

 its cavity, a series of lobes which correspond with the main 

 roots (not the branches) of the vagus and glosso-pharyngeal 

 nerves (PL xvi. fig. 5). There appear to me to be present 

 seven or eight projections : their number cannot however be 

 quite certainly determined. The first of them belongs to the 

 root of the glosso-pharyngeal, the next one is interposed between 

 the glosso-pharyngeal and the first root of the vagus, and is 

 without any corresponding nerve-root. The next five corre- 

 spond to the five main roots of the vagus. For each projec- 

 tion to which a nerve pertains there is a special nucleus of 

 nervous matter, from which the root springs. These nuclei do 

 not stain like the remainder of the walls of the medulla, and 

 stand out accordingly very conspicuously in stained sections. 



The coating of white matter which appeared at the end of 

 stage K, on the exterior of each lateral half of the hind-brain, 

 extends from a point just dorsal to the attachment of the nerve- 

 roots to the ventral edge of the medulla, and is specially con- 

 nected with the tissue of the upper of the two already described 

 projections into the fourth ventricle. 



A rudiment of the tela vasculosa makes its appearance during 

 stage Q, and is represented by the folds in the wall of the 

 fourth ventricle in my figure of that stage (PL XV. fig. 7a, t v). 



The development of the brain in Elasmobranchs has already 

 been worked out by Professor Huxley, and a brief but in 

 many respects very complete account of it is given in his 

 recent paper on Ceratodus^ He says, pp. 30 and 31, ''The 

 development of the cerebral hemispheres in Plagiostome Fishes 



1 Das Geliirn d. Selacliier, Leipzig, 1870. 



■- Proceedings of tlie Zoological Society, 1876, Pt. I. p. 30 and 31. 



