THE BRAIN. 379' 



the anterior, middle, and posterior commissures of the third 

 ventricle. Their development is complicated, and difficult to 

 follow. 



In front of the lamina terminalis, the two hemispheres extend 

 forwards side by side, very close to each other (Fig. 152), but 

 separated by a median cleft in which lies the connective tissue 

 lamina from which the falx cerebri is formed. At the hinder 

 end of this cleft, just in front of the lamina terminalis, the walls 

 of the two hemispheres come in contact and fuse; and from this 

 fused patch, which is somewhat triangular in shape as seen in 

 sagittal section, the commissural bands are formed. The corpus 

 callosum (.Pig. 153, cc), the most important of them, is formed 

 from, the dorsal part of the fused patch ; it develops from before 

 backwards, the anterior end being formed first. The fornix, in 

 which the fibres are mainly longitudinal in direction, is formed 

 from the ventral part of the patch ; and the anterior commissure 

 from its hinder end, just in front of the lamina terminalis. It 

 is not quite clear whether the fifth ventricle (Fig. 153, v) is 

 formed by the breaking down of the central part of the fused 

 patch, or is merely a persistent part of the original cleft between 

 the two hemispheres. 



The surfaces of the hemispheres are at first smooth, and even 

 in the adult rabbit are only slightly convoluted. The convolu- 

 tions arise as foldings or grooves of the surface, extending to a 

 greater or less depth, and are classed as primary or secondary 

 according to whether they are folds involving the whole thickness 

 of the wall of the hemisphere, or mere grooves in its substance. 



The olfactory lobes appear, about the fourteenth day, as a 

 pair of hollow outgrowths from the ventral surface of the anterior 

 ends of the cerebral hemispheres ; by the eighteenth day (Fig. 

 151, by) they have become prominent structures. 



The mid-brain. In the early stages, up to about the twelfth 

 daj' (Fig- 150), the mid-brain is very imperfectly marked oft 

 from the fore-brain ; later on (Fig. 151), the boundary between 

 the two divisions becomes well defined. 



As compared with the chick, the mid-brain of the rabbit is 

 of rather smaller size, and less prominent : it is further dis- 

 tinguished by its tendency to grow backwards over the hind- 

 brain, a tendency already present on the twelfth day (Fig. 150), 



