The origin of the Corpus callosiim, a contrib. upon the cerebral comm. etc. 541 



very small in the Amphibia, intermediate in the Chelouia aud Uphidia, 

 and large in the birds and mammals (see figs. 11, 21. 17 and 6). 



The frontal division of the corpus callosum is well deve- 

 loped in the Amphibia: the presence of the commissura eornu am- 

 mouis is doubtful. It appears as if this commissure arises aud de- 

 velops in connection with the overlapping of tlie thalameucephalon 

 by the hemispheres. In the Chelouia the commissura cornu am- 

 monis is about \ s the size of the frontal division aud forms a large 

 distinct tract. In other Sauropsida these divisions are also observed. 

 In the birds they are both very diminutive. In the lower mam- 

 malia the commissura cornu ammouis is as large, or larger, than the 

 frontal division of the cornu ammouis. Above this group, the fron- 

 tal division extends rapidly, in distribution area until it greatly sur- 

 passes all the other divisions of the cerebral commissures combined. 



Explanation of Plate. 



Plate XXV. 



Illustrating the origin and development of the corpus callosum. 

 AbbreYiatious. 



hem commissure above optic chiasma, connecting the region of the 

 peduncles (basilar commissure, Meynert?), 



cal corpus callosum, 



cal' posterior division of the corpus callosum or commissura cornu 

 ammonis, 



cbl cerebellum, % 



ohm optic chiasma, 



acm anterior commissure, 



cor fibres of the corona radiata, 



est corpus striatum, 



fm foramen of Monro, 



fmh fimbria hippocampi, 



for columnae of the fornix, 



hem cerebral hemispheres, 



It lamina termiuaiis, 



olf olfactory lobes, 



j)ch choroid plexus of the lateral ventricles, 



pcm posterior commissure, 



ped cerebral peduncles, 



p.fr pars frontalis of the anterior commissure, 



3Ü* 



