204 



plies a large part of tlie ventro-lateral region of the hemispheres, which 

 in the higher Mammals is supplied by the corpus callosum. 



The position of the earliest fibres of the corpus callosum, in the 

 Mammalian Embryonic brain is directly above the anterior commissure 

 although separated by a considerable interval of the lamina terminalis, 

 in which descend the fibres of the fornix. The corpus callosum has 

 the same position permanently in the adultMonotremes and Marsupials. 



There is no ground for comparing the upper bundle with the fornix 

 as the distribution of its fibres is altogether different. The conclusions 

 at present drawn from the above observations are therefore the following: 



1 . That the lower bundle in the Reptiles, Amphibia and Fishes 

 represents the Anterior Commissure. This commissure consists 

 primitively of the, a) Pars olfactoria, with extensive commissural 

 fibres of the ventro-lateral moiety of the hemispheres superadded, 

 b) Pars temporalis, which, feebly developed in the Amphibia, increases 

 with the progressive development of the temporal lobe. 



2. That the upper bundle in the Amphibian and Eeptilian Brain 

 is the Corpus Callosum and contains the fibres of the dorso-medial 

 moiety of the hemispheres. 



Histologisch-anatomisches Laboratorium, München, 28. Januar 1886. 



Postscript. (16. Februar.) 



There is considerable discrepancy in the description of the cere- 

 bral commissures in various Teleosteans by Mays er, F ritsch and 

 Rabl-Rückhard. After a careful study of these commissures in the 

 brain of the carp it strikes me that this may be due to a high degree 

 of generic specialization in the Teleostean brain rather than to imper- 

 fect observation. For example I find the commissures in the carp differ 

 very widely from those in the Trout. From this fact it seems somewhat 

 hazardous to homologize these bundles in the Teleostean brain with 

 the upper and lower bundles which are so distinct in the Amphibian 

 and Reptilian brain. Yet, omitting the fornix, the generalization of 

 Miklucho-Maclay is probably correct that the extensive commissures 

 in the fish brain present the anterior commissure and corpvis callosum 

 in their primitive form. 



The Pars temporalis of anterior commissure is very well developed 

 in the brain of the Turtle, and from its horizontal course is more 

 readily demonstrated than the Pars olfactoria. The distribution of the 

 upper bundle (corpus callosum), over the entire inner wall of the 

 heniispheres can be beautifully shown in vertical sections. Opposite 

 the upper part of the foramen of Monro, the upper bundle gives off 

 a large branch which passes above the foramen along the inner fold 



