444 C. JUDSON HERRICK 



in this phylum was at the caudal end of the endbrain rather than 

 adjacent to the terminal plate, as in most other fishes. 



The initial functional impulse for the difTerentiation of the 

 endbrain was undoubtedly the excitations arising in the nasal 

 epitheUum, and the precocious evagination of the region of the 

 posterior pole in the Amphibia may be explained, as just sug- 

 gested, by a more caudal position of the nasal sac in the precursors 

 of this phylum than in most types of fishes, or there may be some 

 other factor not yet recognized. 



TERMINAL PLATE 

 COMMON VENTRICLE 



.NASAL SAC 



EREBRAL HEMISPHERE 



■LATERAL VENTRICLE 

 BETWEENBRAIN 



Fig. 17 Diagram of a horizontal section through the forebrain of fetal Lepido- 

 siren (Graham Kerr's stage 30), illustrating the relations of the nasal sacs to 

 the hemispheric evaginations. Olfactory nerve fibers are seen connecting the 

 nasal sacs with the posterior parts of the evaginations of the lateral walls of the 

 endbrain. Redrawn from Elliot Smith ('08, fig. 4). 



FOREBRAINS OF MUDFISHES 



As we have seen, the most characteristic difference between 

 the forebrains of the Amphibia and those of the various groups 

 of highly speciahzed fishes lies in the more complete evagination 

 of the primitive endbrain into the cerebral hemispheres in the 

 Amphibia. And we next inquire what were the reasons — adap- 

 tive, physiological, mechanical, or other — leading up to this 

 departure from the directions taken by the higher fishes. 



First of all it should be borne in mind that the movement in 

 this direction was not confined to the Amphibia. We find to-day 

 fully evaginated hemispheres in two species of lungfishes. Prob- 

 ably the trend in this direction began in the ganoidean stem 

 ancestral to both Dipnoi and Amphibia. 



