The origin of the Corpus Callosum, a contrib. upon the Cerebral Coium. etc. 239 



served to unite and pass forwards together. The meaning of the 

 division is not clear. As far as I can determine this bundle is a true 

 commissure. 



The longest transverse axis of the hemispheres in Ran a is placed 

 vertically while in the Urodeles it is oblique (cf. figs. 14 and 8); the 

 medial hemisphaeral surface in the Batrachia must therefore be 

 regarded as equivalent to the dorsal surface in the Urodela and 

 this area may be described as the dorso- medial wall of the lateral 

 ventricle. This wall in Ran a contains two masses of cells, an 

 upper mass isi) of larger cells which extends from behind the fora- 

 mina of Monro and diminishes as the sections pass forwards, and a 

 lower mass (n) of smaller more compact cells which increases as 

 the sections pass forwards. It is important to observe these distinc- 

 tions carefully as they enable us to distinguish the hemispheres from 

 the olfactory lobes (fig. 12). The upper bundle passes forward, close 

 to the ventricle, between these two cell groups (fig. 9 and 10) distri- 

 butings all of its fibres to the upper group. 



Fiff. 2. 



Sagittal .section of the braiu of It an a. cbl ceretelluin. cc corpus callosum. acm' — " anterior com- 

 missure, pars olfactoria and pars temporalis, chm optic chiusma. ilc 'M^ ventricle, fm foramen of 

 Monro, hph hypophysis, inf infundibulura. mf medullary folds, mtc 4** ventricle, msc iter. 

 pan posterior commissure, pn pineal stalk, encephalic portion, sew superior commissure, th optic 

 thalamus, vc ventriculus communis. 17, II, I cranial nerves. 



In Ran a and Sal am and r a the bundle has an ascending course. 

 In the Urodeles, where the upper group is larger and the hemispheres 

 are separated above, the bundle descends and is scattered over the 

 whole of the dorso-medial wall (fig. 15). As the bundle lies below 

 the upper group, the upward distribution of its fibres is best seen 

 in transverse sections, in front of the foramina of Monro, where the 

 fibres pass in delicate fasciculi between parallel rows of cells, the 

 cell processes extending at right angles to the direction of the 

 bundles (fig. 9) . The bundle finally spreads out and terminates in an 

 expansion of the cells of the upper group. No fibres of this bundle 



