THE FORM AND SUBDIVISIONS OF THE BRAIN 25 



foramina open. This results in some radical differences from reptilian 

 and mammalian arrangements of the related fiber tracts and mem- 

 branous parts, as elsewhere described (p. 291; '35). The third ven- 

 tricle is expanded dorsally into the complicated membranous paraph- 

 ysis and dorsal sac. Ventrally, the great elongation of the preoptic 

 nucleus gives rise to a large preoptic recess between the anterior com- 

 missure ridge and the chiasma ridge, and in front of the latter there is 

 a lateral optic recess (fig. 96), which in early larval stages extends 

 outward as far as the eyeball, as a patent lumen of the optic nerve 

 ('41), an arrangement which persists in the intracranial part of the 

 nerve of adult Necturus ('41a). In the hypothalamus the ventricle is 

 dilated laterally ('35a, p. 253; '36, figs. 10-14), and posteriorly it is a 

 wide infundibulum with membranous roof and thin but nervous floor 

 and posterior wall. The latter is the pars nervosa of the hypophysis 

 and is partly enveloped by the pars glandularis (figs. 2, 101; '35a, p. 

 254; '42, p. 212 and figs. 56-65; Roofe, '37). The aqueduct of the 

 midbrain is greatly expanded dorsoventrally. Its ventral part is con- 

 tracted laterally by the thick peduncles and tegmentum, and the 

 dorsal part is dilated as an optocoele. The sulcus lateralis mesenceph- 

 ali marks its widest extent, and tectal structure reaches far below this 

 sulcus. The fourth ventricle is of typical form except anteriorly, 

 where the wide lateral recess with membranous roof extends outward 

 and forward to cover the whole dorsolateral aspect of the auricular 

 lobe (figs. 90, 91 ; '24, p. 627). The rhombencephalic chorioid plexus is 

 elaborately developed in interesting relation with the peculiar endo- 

 lymphatic organs of this animal ('35, p. 310). The ventricular sys- 

 tems of adult Triturus (Diemyctylus) and of larval and adult stages 

 of Hynobius have been described and illustrated with wax models 

 bySumi('26, '26a). 



The ventricular surface of both larvae and adults is clothed with 

 very long cilia. These are not preserved in ordinary preparations and 

 in our material are seen only in Golgi sections, where their impregna- 

 tion is erratic and local ('42, p. 196). They are most frequently seen 

 in the infundibulum and optocoele under the tectum. In the vicinity 

 of the posterior commissure the ciliated ependyma is thickened (sub- 

 commissural organ of Dendy), and to it the fiber of Reissner is at- 

 tached ('42, p. 197). This thick, nonnervous fiber extends backward 

 through the ventricle to the lower end of the spinal cord and, like the 

 cilia, is apparently an outgrowth from the internal ependymal mem- 

 brane. 



