XERVOUS SYSTEM OF THE VERTEBRATA. 361 



third ventricle round the end of the brain to the optic chiasma, is the 

 rudiment of the lamina terminalis (figs. 257 It and 255 trm). Up to 

 this point the development of the cerebrum is similar in all Verte- 

 brata, but in some forms it practically does not proceed much further. 



In Elasmobranchii, although the cerebrum reaches a considerable 

 size (fig. 254 cer), and grows some way backwards over the tbalamen- 

 cephalon, yet it is not in many forms divided into two distinct lobes, 

 but its paired nature is only marked by a shallow constriction on the 

 surface. The lamina terminalis in the later stages of development 

 grows backwards as a thick median septum which completely separates 

 the two lateral ventricles 1 (fig. 268). 



There are, it may be mentioned, considerable variations in the 

 structure of the cerebrum in Elasmobrauchii 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 ch) 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 deep- 

 ening 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 commissure, 

 and in the higher members of the group, especially Aves (fig. 250), 

 the hemispheres may obtain a considerable development. 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 com- 

 plicated 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 261 / and 258 i). From this 

 septum is formed the falx cerebri and other parts. 



1 A comparison of the mode of development of this septum with that of the septum 

 lucidum with its contained commissures in Mammalia clearly shews that the two 

 structures are not homologous, and that Miklucho-Maclay is in error in attempting to 

 treat them as being so. 



