THE ORGANS OF THE OUTER GERM-LAYER. 441 



furrows that cut into the cortex of the cerebrum more or less deeply 

 from the outside, but cause no corresponding internal projections in 

 the wall of the ventricle. 



As regards its general features, the embryonic growth of the cerebral 

 vesicles is especially characterised by an enlargement backward. In 

 the third month the posterior lobe already completely overlies the 

 optic thalamus (fig. 242) ; in the fifth month it begins to extend over 

 the corpora quaclrigemma (fig. 241), which it entirely covers up in 

 the sixth month. From there it spreads over the cerebellum 

 (fig. 256). The cerebrum is not characterised in all Mammals by 

 such an extraordinary growth as in Man ; comparative anatomy 

 teaches rather that the stages of development of the human brain in 

 different months here described, are met with in other Mammals as 

 permanent conditions. 



In some animals the posterior margins of the hemispheres extend as far as 

 the corpora quadrigemina ; in others they cover these more or less completely ; 

 in others, finally, they have grown over the cerebellum more or less. On the 

 whole, the increase in the volume of the cerebrum, which is so varied in 

 Mammals, goes hand in hand with an increase in intelligence. 



The vesicles of the hemispheres undergo additional complication 

 (in Man in the course of the second and third months), owing to 

 infoldings of their thin walls, which still enclose a large cavity. As 

 a result of this there arise on the outer surface deep furrows, which 

 separate large areas from one another and which have been designated 

 as total furrows or fissures by His, who has rightly estimated their 

 importance in the architecture of the brain. Corresponding to the 

 f urrows which are visible on the outer surface, there are more or less 

 prominent elevations on the inner surface of the lateral ventricles, 

 by means of which the latter become narrowed and reduced in size. 

 The total furrows of the cerebral hemispheres are the fissure of 

 SYLVIUS (fossa Sylvii), the arcuate fissure, embracing the hippo- 

 campal fissure (fissura hippocampi), the fissura choroidea, the fissura 

 calcarina, and the fissura parieto-occipitalis. The elevations produced 

 by them are called the corpus striatum, fornix and pes hippocampi, 

 tela choroidea and calcar avis. A prominence which in the embryo 

 corresponds to the fissura parieto-occipitalis, becomes obliterated in 

 the adult by a considerable thickening of the wall of the brain, so 

 that no permanent structure results from it. 



The fissure of SYLVIUS (fig. 252 Sy.g) is the first one formed. It 

 appears as a shallow depression of the convex outer surface at about 



