404 DEVELOPMENT OF ELASMOBRANCH FISHES. 



fig. &/, //), and correspond in position with the fasciculi teretes 

 of human anatomy. Part of the root of the seventh nerve 

 originates from them. They project freely in front into the 

 cavity of the fourth ventricle (PI. 16, 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 (PL 16, fig. 7 a, 

 rt), which are regarded by Miklucho-Maclay 1 as representing the 

 true cerebellum. 



By stage O 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 

 (PI. 17, 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 correspond to the 

 five main roots of the vagus. For each projection 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 (PI. 16, fig. ja, tv). 



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 



1 Das Gehirn d. Selachier, Leipzig, 1870. 



