The origin of the Corpus Callosum, acontrib. upon the Cerebral Comm. etc. 237 



In the batrachia this ventricle is much contracted and the cerebral 

 commissures are placed midway in the lamina terminalis, the sum- 

 mit only of the commissural ridge projecting. An important character 

 is that the cerebral commissures in the Amphibia lie behind the 

 foramina of Monro. We find further that, with the closing of the 

 ventriculus communis, there are a series of stages represented 

 by Menopoma, Menobranchus &c, diverging from the fish 

 type, towards the reptilian type, w^hich is approached in the Ba- 

 trachia. 



The condition of the cerebral commissures in the Amphibia is 

 generally that of two compact bundles lyiug in the same vertical 

 plane one arching upwards towards the mantle, the other downwards 

 into the brain stem (fig. 8). In most of the Urodeles the upper 

 bundle forms the upper part of the commissural ridge and is covered 

 with a thin layer of ependyma cells. 



In Menobranchus however, the upper bundle is completely 

 separated from the lower (fig. 13), and crosses the ventricle indepen- 

 dently, so that a fold of the vascular plexus of the pia, which in 

 this genus is greatly developed passes through the interspace. In the 

 batrachia the cerebral commissures are also separated, but they lie 

 in the vertical portion of the lamina terminalis, which is here thickened 

 by the union of the inner wall of the hemispheres. 



The lower bundle, in the Amphibia, is of lesser diameter than 

 the upper. In Ran a the proportion is about 2 : 4, in the Urodeles 

 it is about 2:3. In Kana there is a well marked division of this 

 bundle into a large upper and small lower and posterior tract (fig. 7), 

 which seem to be quite distinct. The upper tract bends downwards 

 and forwards upon either side and scatters widely in a mass of cells. 

 These cells form a nucleus in the centre of what is considered the 

 corpus striatum area. The scatteriug of the fasciculi in this nucleus 

 is misleading and previous investigators have described this tract as 

 terminating here and forming a commissure of the corpora striata. 

 It is probable that many of the fibres do terminate here, this I can- 

 not determine, but a considerable part is observed to traverse this 

 mass of cells and bending downwards pass forwards in the ventro- 

 lateral wall of the hemispheres, parallel with the lateral peduncular 

 tracts (fig. 11). This course may be followed in successive horizontal 

 sections from above downw^ards, but is by no means an easy matter 

 to determine. The fact that this tract is not simply a commissure 

 of the corpus striatum is to my mind demonstrated in the Urodeia, 



