446 EMBRYOLOGY. 



the anterior end of the fissura calcarina in a vertical direction to 

 the bent rim of the mantle (Mantelkante), and sharply separates the 

 occipital and parietal lobes from each other. 



A third factor of great importance in the development of the 

 cerebrum is the formation of a system of commissures, which sup- 

 plements the connection of the two cerebral vesicles, at first effected 

 by the embryonic lamina terminalis only. Those investigators who 

 have occupied themselves with these difficult matters assert that in 

 the third embryonic month fusions take place between the facing 

 median walls of the hemispheres. These fusions begin in front of 

 the foramen of MONRO within a triangular area. The fusions in this 

 region occur only at the periphery, not in the middle of the area. 

 Three parts of the brain of the adult are thus produced : in front, the 

 genu of the corpus callosum, behind, the columns of the fornix, and 

 between them, the septum pellucidum ; the latter contains a fissure- 

 like cavity, in the region of which the contiguous walls of the hemi- 

 spheres, here very much attenuated, have remained separated from 

 each other. Consequently the cavity just mentioned the ventriculus 

 septi pellucidi [or fifth ventricle] ought not to be placed in the same 

 category with the other cavities of the brain ; for while the latter are 

 derived from the central canal of the embryonic neural tube, the 

 former is a new production, which has arisen by the enclosure of a 

 portion of the space lying outside the brain between the two hemi- 

 spheres the narrow interpallial fissure. 



A further enlargement of the commissural system is accomplished 

 in the fifth and sixth months. The fusion now proceeds still 

 farther, advancing from in front backwards, and involves that region 

 of the median walls of the hemispheres which, situated between the 

 arcuate fissure [above] and the fissure of the choroid plexus [below], 

 has already been described as the marginal arch (Randbogen). By 

 fusion of the anterior part of the marginal arch with its fellow of the 

 opposite side, which process takes place as far as the posterior limit 

 of the between-brain, there arise the body of the corpus callosum 

 and the splenium, as well as the underlying fornix. The furrow 

 bounding the corpus callosum above (sulcus corporis callosi) is there- 

 fore the anterior part of the arcuate furrow, whereas the posterior 

 portion, that of the temporal lobe, is subsequently known as the 

 iissura hippocampi. 



The structure of the cerebrum is completed by the appearance of 

 numerous cortical furrows. These differ in rank from the total furrows 

 already described, because they are confined to the outer surface of the 



