ORIGIN OF THE CEREBRAL HEMISPHERES 



439 



The forebrain of the lamprey (fig. 16) suggests in its form an 

 approach to the selachian condition, though the resemblance is 

 not close. The olfactory organ is large and the olfactory bulb 

 is fully evaginated. The cerebral hemisphere contains behind 

 the bulb a smaller ollactory lobe whose homologies in other 

 vertebrates are by no means clear; resemblance to the lateral 

 olfactory nucleus complex of higher forms is suggested, though 

 this homology is not certain. The primitive endbrain and 

 betweenbrain are in very generaUzed condition, though the more 



Fig. 14 Diagrammatic longitudinal section of the forebrain of an amphibian. 

 The larger part of the endbrain has been evaginated to join the olfactory bulbs 

 in the formation of the hollow cerebral hemispheres, whose walls are nowhere 

 greatly thickened. 



Fig. 15 Cross-section through the amphibian cerebral hemispheres m front 



of the terminal plate. 



important regions of higher forms can be identified (Johnston, 

 '02, '12; Herrick and Obenchain, '13). 



The feature of the cyclostome brain of greatest interest from 

 the standpoint of this sketch is the histological structure. This 

 is exceedingly primitive and generahzed. Throughout the brain 

 the cell bodies of the neurons are in the original embryonic 

 position as a layer of central gray matter bordering the ventricular 

 cavities. Their dendrites and axons are directed outward, where 

 they form a superficial layer of white matter within which all 



