OEIGIN OF THE CEEEBRAL HEMISPHERES 431 



part of this thickened region will evaginate to form hollow olfac- 

 tory bulbs (as in figs. 3 and 4).i 



Figure 3 illustrates in a schematic way a condition of the fore- 

 brain which may be taken as a point of departure for a consid- 

 eration of the diverse directions taken in the further evolution of 

 this region in different vertebrates. 



Here the endbrain has departed but httle from the primitive 

 relations of a simple neural tube save for the evagination of an 



TERMINAL PLATE 

 NASAL SAC 

 OLFACTORY BULB 



COMMON VENTRICLE 

 PRIMITIVE ENDBRAIN 



Fig. 3 Diagram illustrating the probable primitive vertebrate type of fore- 

 brain as seen in longitudinal section. A portion of the endbrain behind the ter- 

 minal plate has evaginated in correlation with the differentiation of the adjacent 

 nasal organ and olfactory nerve. This forms the olfactory bulb which comprises 

 the entire cerebral hemisphere. The cavity of the olfactory bulb is the lateral 

 ventricle. The remainder of the endbrain is unevaginated (primitive endbrain). 



In this and the following diagrams the cut surfaces of the walls of the neural 

 tube are conventionally marked, as follows: primitive endbrain, diagonal lines; 

 evaginated endbrain (cerebral hemispheres), horizontal lines; betweenbrain, 

 vertical lines; midbrain, unshaded. 



olfactory bulb on each side close behind the terminal plate. 

 This evaginated portion of the neural tube lies closely apposed 

 to the nasal sac. In this diagram we can recognize two sub- 

 divisions of the endbrain: 1) an unevaginated portion which may 

 be called the telencephalon medium or primitive endbrain, and 



1 In this connection I am not unmindful of the very atypical development of 

 the central nervous system of teleosts in early stages; but the originally solid 

 nervous cord is early transformed into a neural tube whose subsequent develop- 

 ment is comparable with that of other vertebrates. 



