438 C. JUDSON HERRICK 



than those of other fishes (in fact, probably from some points of 

 view less so in these species), nevertheless has potentialities of 

 further difTerentiation from which the other types are apparently 

 forever excluded. 



The paleontological evidence seems to be clear that the ances- 

 tors of the Amphibia were primitive ganoids of the general type 

 illustrated by Polypterus and that the Dipnoi arose from a closely 

 related stock. The forebrains of the existing members of these 

 groups are all characterized by thin nervous walls and elaborately 

 developed membranous parts. But little is known of their 

 histological structure, but evidently this is very simple and 

 primitive. The forebrains of Polypterus and Ceratodus have 

 deviated in form from the primitive pattern in directions very 

 divergent from that shown by the Amphibia. On the other 

 hand, the resemblance of Protopterus and Lepidosiren in this 

 respect to the generalized Amphibia is very close. It is, there- 

 fore, very probable that the primitive ganoidean ancestor of 

 amphibians had a forebrain closer to that of Protopterus than 

 of any other surviving type. 



Turning now to the brains of the Amphibia (figs. 14, 15), 

 the resemblance between their forebrains and those of Proto- 

 pterus and Lepidosiren is very close indeed. In the relatively 

 thin-walled cerebral hemisphere the various correlation centers 

 of the endbrain, which serve primarily as apparatus of the differ- 

 ent forms of response to olfactory stimuli and for compounding 

 these with other forms of sensory excitation, may enlarge without 

 crowding by simply expanding the size of the hemispheric vesicle. 

 And this thej^ have done, to a notable degree in the Amphibia 

 and in still larger measure in the classes which have been derived 

 from them. 



THE CYCLOSTOME FOREBRAIN AND ITS HISTOLOGICAL STRUCTURE 



The cyclostomes occupy a peculiar position in the vertebrate 

 series. Though undoubtedly aberrant, they probably arose 

 from extinct types which were close to the roots of the vertebrate 

 genealogical tree. Their brains will next be considered. 



