The origin of the Corpus callosum, a coiitrib. upon the cerebral comm. etc. 537 



large structure. Studied in horizontal as well as in sagittal and 

 transverse sections it is found to enter laterally into three distinct 

 divisions. One extends backwards as the pars temporalis, the se- 

 cond extends forwards into the olfactory lobes. A third, branching 

 from the second, is quite distinct in its distribution from the former. 

 For this division, already observed by Flower, Sander and others, 

 and shown to exist on i)hysiological grounds by Ganser I propose 

 the name pars frontalis. This title gives an approximate idea 

 of the distribution of this bundle to a region of the mantle , whicli 

 is quite distinct from that supplied either by the pars temporalis or 

 olfactoria. Above the anterior commissure is the corpus callosum, 

 now about V4 ^^ large. We first observe that these commissures are 

 separated by a very narrow interval, in contrast with their wide 

 separation in the sheep's brain. At the sides of the corpus callosum 

 are the colums of the fornix. The relations of these tracts are unique. 

 The callosal and fornix fibres ascend together in the inner ventri- 

 cular wall and then divide into two tracts. At this stage one of 

 these tracts cal extends forwards and upwards below the faint hip- 

 pocampal sulcus. The other tract cal' extends backwards above 

 the foramen of Monro into the hippocampal region. 



A later stage, of a specimen 40 mm in length , is represented 

 in fig. 25, taken in a plane anterior to the fornix columns. Here 

 the corpus callosum has a diameter equal to that of the anterior 

 commissure. Dorsally, its two divisions c«/ and cal' are still more 

 clearly defined than before. The corpus callosum sends a few fibres 

 upwards into the inner mantle. The fibres of the pars frontalis 

 now extend over the roof of the ventricle into a region of the mantle 

 which is supplied by the corpus callosum in the higher mammals. 



The marsupial brain thus gives us the typical development of 

 the anterior commissure. I find that it also presents a generalized 

 type in the relations of the corpus callosum which enables us to 

 interpret both the reptilian and higher mammalian conditions of this 

 commissure. This is partly owing to the fact that the hippocampal 

 sulcus persists in the inner wall of the lateral ventricle into adult 

 life, instead of disappearing in the later stages of development as it 

 does in the higher mammalia. 



In a specimen of 75 mm in length the distance between the 

 corpus callosum and anterior commissure has increased by the ex- 

 tension of the terminal plate. The hippocampal fold is well de- 

 fined into several layers of gray, white and granular substance which 



