ii Journal of Comparative Neurology. 



Dr. Cunningham does not agree with Mihalkovics and other 

 writers that the cephahc part of this fissure produces the callosal 

 fissure but regards this portion as transitory. 



The remainder of the complete fissures are transitory. The 

 mesal wall of the hemispheres being thicker than the outer the tran- 

 sitory fissures there first appear. They may appear as early as the 

 eighth week and reach a high degree of development only after the 

 tenth week. Their maximum development is reached between the 

 periods when the fornix and callosum appear, i.e. near the beginning 

 of the fifth month. 



On the mesal aspect of the hemispheres the transitory fissures, 

 which vary in number, radiate from the arcuate fissure toward the 

 free border of the hemisphere and others, lying between them but not 

 connecting with the arcuate fissure, appear near the margin. The 

 usual number on each hemisphere appears to be eight. 



The primitive fissures which bound the cuneus are the pre- 

 cursors of the calcarine and parieto-occipital fissures. As the wall of 

 the cerebral vesicle thickens, and the hemisphere elongates, the 

 stellate fissures become detached from the arcuate fissure and gradu- 

 ally disappear. 



Upon the outer aspect the arrangement of the fissures is also 

 radial, with the Sylvian fossa as the centre, but these fissures usually 

 do not reach the fossa. There is frequently an additional fissure oc- 

 cupying the place of the future Sylvian. In some cases the pre- 

 cursor of the calcarine is carried horizontally around the occipital 

 pole, appearing on the lateral surface as an external calcarine fissure. 



It is regarded probable that under abnormal conditions, as when 

 the callosum fails to develop, the transitory fissures may persist to a 

 certain extent. The fact emphasized by Sir William Turner that in 

 Macropus, where the callosum is rudimentary, the radiating fissures 

 resemble those of an immature human brain is at least suggestive 

 that the development of the callosum has some effect on the disap- 

 pearance of these fissures. 



Dr. Cunningham fully agrees with His and Koelliker that the 

 occipital lobe is due to a general growth and not a local out-pouching 

 of the hemisphere, its peculiar form is due to the transformations pro- 

 duced by axial flexures — especially the pons flexure. This backward 

 thrusting of the posterior part of the hemisphere is chiefly responsi- 

 ble for the preservation of the precursory calcarine and parieto- 

 occipital fissures. The temporary fissures are in all probability 



