CEREBRAL COINIMISSURES IX THE VERTEBRATA. 



493 



naturally suggests the possil)ility that the caudal or post-portal part of tlic choroidal fold 

 in the Mammal, which also links the hippocampus to the optic thalamus (tig. 27), may 

 have arisen in a similar manner. 



The relations of the optic thalamus to the cerebral hemisphere have Ikhmi elucidated 

 within recent years hy the researches of His, Lachi, and Hoclistetter. 1 wish briefly to 

 call attention here to certain features in tliis relationship which these writers have 

 not emphasized, and which afford the })ro]mble explanation of the presence of the 

 commisswa aberrant in such a peculiar locality. 



From the accompanying rough sketch, which is slightly modified from one of Hiss 

 drawings *, it will be conceivable that the optic thalamus extending forward as the 



Fio;. 29. 



teL 



Rough sketch of a mesial view of the forebraiii of a human embryo uf 10'2 mm. neck-length. 



Mainly after His. 



lateral wall of the third ventricle, becomes directly continuous on the ventral side with 

 the corpus striatum at /3, whereas its dorsal part becomes continuous at a, not with the 

 ganglionic mass, but with the mantle (using this term in the original sense of lleichert). 



The corpus striatum in the floor of the hemisphere may be seen extending through the 

 widely ojien foramen of jMonro at /3 to fuse witli the lower part of the optic thalamus. 

 But this figure is introduced more especially to demonstrate the relationship which 

 exists between the dorsal portion of the optic thalamus and the mantle of the hemisphere 

 at «. Tlie significance of this relationship will be rmderstood when it is remembered tliat 

 it is this '• parathalamic " area of the mantle which becomes modified to form the caudal 

 portion of the liippocampus, au'd that it is the connecting band between the hippocampus 

 and the thalamus which ])ecomes attenuated (according to the hypothesis advanced earlier 

 in this account) and is then converted into the caudal part of the lateral choroid plexus. 



Bearing in mind these primitive relations of the thalamus to the eerebi-al hemisphere, 

 which obtaiu at some epoch in the development of all Vertebrates and persist into adult 

 lile in the lehthyopsida, the course of the fil)res of the commissnra aOerrans becomes 

 intelligible. For it springs from the parathalamic mantle, wliich becomes the hippo- 



* " Formentwick. des llenschl. Vorderhims." AbhaniU. d. koiiigl. Siiclis. Uesell. d. Wissensth., Bd. xv. 2\o. viii. 

 1889. 



