ORIGIN OF THE CEREBRAL HEMISPHERES 437 



In the lungfishes, on the other hand, there is a tendency for 

 the entire endbrain behind the olfactory bulbs to evaginate 

 laterally. In Ceratodus (Bing and Burckhardt, '05) the form 

 suggests that of the selachian forebrain, though with more exten- 

 sive lateral outpouching, with less thickening of the walls in the 

 vicinity of the terminal plate, and with many other differences. 

 The thin lateral walls of the primitive endbrain are bowed out- 

 ward (not everted) and, as in Polypterus, they are connected 

 with each other dorsally for their entire length by a plexiform 

 membrane, thus enclosing a wide common ventricle. The 

 lateral outpouching (exclusive of the olfactory bulb) extends 

 but little, if at all, rostrally of the terminal plate and there is 



Fig. 13 Cross-section through the cerebral hemispheres of Protopterus be- 

 tween the olfactory bulbs and the terminal plate. Redrawn from Burckhardt 

 ('92, fig. 23). 



no sagittal fissure separating two cerebral hemispheres behind 

 the olfactory bulbs. The true (fully evaginated) cerebral 

 hemisphere, accordingly, contains only the olfactory bulb. 



The brains of Lepidosiren and Protopterus (figs. 12, 13) 

 resemble each other closely, and in these cases the larger part of 

 the side wall of the primitive endbrain has joined the olfactory 

 bulb to form a fully evaginated cerebral hemisphere. This 

 results in the formation of extensive, hollow, thin-walled hemi- 

 spheres separated by a deep sagittal fissure, whose rostral ends 

 are formed by the olfactory bulbs and whose remaining parts 

 include tissues which in most other fishes are represented in the 

 unevaginated primitive endbrain. These evaginations are pro- 

 truded forward far beyond the terminal plate. 



This type of brain with extensive hollow cerebral hemispheres, 

 though perhaps no more efficient as a neuromotor apparatus- 



