NERVOUS SYSTEM OF THE VERTEBRATA. 439 



the structure of the cerebrum in Elasmobranchii into which it is 

 not however within the scope of this work to enter. 



In the Teleostei the vesicles of the cerebral hemispheres 

 appear at first to have a wide lumen, but it subsequently 

 becomes almost or quite obliterated, and the cerebral rudiment 

 forms a small bilobed nearly solid body. In Petromyzon (fig. 

 253 c/i) the cerebral rudiment is at first an unpaired anterior 

 vesicle, which subsequently becomes bilobed in the normal 

 manner. The walls of the hemispheres become much thickened, 

 but the lateral ventricles persist 



In all the higher Vertebrates the division of the cerebral 

 rudiment into two distinct hemispheres is quite complete, and 

 with the deepening of the furrow between the two hemispheres 

 the lamina terminalis is carried backwards till it forms a thin 

 layer bounding the third ventricle anteriorly, while the lateral 

 ventricles open directly into the third ventricle. 



In Amphibians the two hemispheres become united together 

 immediately in front of the lamina terminalis by commissural 

 fibres, forming the anterior commissure. They also send out 

 anteriorly two solid prolongations, usually spoken of as the 

 olfactory lobes, which subsequently fuse together. 



In all Reptilia and Aves there is formed an anterior commis- 

 sure, and in the higher members of the group, especially Aves 

 (fig. 250), the hemispheres may obtain a considerable develop- 

 ment. Their outer walls are much thickened, while their inner 

 walls become very thin ; and a well-developed ganglionic 

 mass, equivalent to the corpus striatum, is formed at their 

 base. 



The cerebral hemispheres undergo in Mammalia the most 

 complicated development. The primitive unpaired cerebral 

 rudiment becomes, as in lower Vertebrates, bilobed, and at the 

 same time divided by the ingrowth of a septum of connective 

 tissue into two distinct hemispheres (figs. 260 and 26 \f and 

 258 I). From this septum is formed the falx cerebri and 

 other parts. 



The hemispheres contain at first very large cavities, com- 

 municating by a wide foramen of Munro with the third ventricle 

 (fig. 260). They grow rapidly in size, and extend, especially 

 backwards, and gradually cover the thalamencephalon and the 



