The origin of the Corpus Callosum, a contrib. upon the Cerebral Comm. etc, 249 



spx supraplexus, extra ventricuhir portion of the the plexus choroi- 



deus (formerly mistaken for the pineal gland), 

 si upper median cell area of the hemispheres in the amphibia, 

 th thalamencephalon'. 



Cyprina carpio. 



Fig. 1. Sagittal section of the ridge uniting the hemispheres, in the floor of 

 the ventriculus communis. The relations of this ridge, in the horizon- 

 tal plane are shown in figs. 2 , 3 and 4 and, in the transverse plane, 

 in fig. 5. The lowest division of the interlobular Commissure, c?n , 

 is exceedingly tine and can only be recognized in vertical section after 

 its position has been observed in transverse sections. The other com- 

 missures are readily observed as represented in the figure. The 

 distinction between an'" and an"" is both in the distribution and size 

 of the fibres. 



Fig. 2. Horizontal section through the middle of the ridge. The olfactory 

 commissure o.a)i distributes its fibres in the base of the olfactory 

 lobes. The internal olfactory tract ol.i forms a chiasma in front of 

 the commissure and its fibres pass forwards into the olfactory nerve 

 bundle. 



Fig. 3. In this horizontal section are seen a portion of the fibres of the 

 internal olfactory tract, also the distribution of the middle commissu- 

 ral bundle, an" into the hemispheres and among the ascending bundles 

 of the cerebral peduncles. 



Fig. 4. This horizontal section at the top of the interlobular ridge shows the 

 distribution of the two portions of the upper division of the commis- 

 sura interlobularis. It seems evident that cm'"' is a distinct bundle 

 which has only a local relation with cm'". 



Fig. 5. Transverse section through the posterior half of the ridge, cutting 

 through a part of cm" and cm"" and through the centre of cm'". 



Cavia. 



Fig. 6. Horizontal section of the brain of an embryo Guinea Pig, correspond- 

 ing approximately to the human brain in the fourth month of deve- 

 lopment (Mihalkovics). This shows very clearly the relations of the 

 anterior commissure as consisting of an apparently single bundle in 

 the centre , but laterally dividing into an upper and forward branch, 

 the pars olfactoria, and lower and backward branch, the pars 

 temporalis. The latter lies somewhat above the cerebral peduncles. 



Rana mugiens and esculenta. 



Fig. 7. Vertical section through the lamina terminales and area of the 3rd 

 ventricle in Rana mugiens. In the lamina terminalis, which is here 



1 In former papers, upon the Amphibian brain, I have employed the brain 

 terminology introduced by Prof. Wilder of Cornell University, which has many 

 advantages. In this paper I retain the older terms in order to avoid confusion, 

 as the reference to former papers is very frequent. 



