372 J. B. JOHNSTON 



medial olfactory nuclei came to be connected by a median mass 

 through which the commissures crossed from side to side. The 

 median mass was called, by Elliot Smith the commissure bed 

 and the whole body, including the paired gray masses, was called 

 the paraterminal body. Second, the enlargement and arching 

 up of the corpus callosum caused the stretching of the lamina 

 terminalis and the commissure bed. The space within the con- 

 cavity of the callosal arch was filled by a part of the paratermi- 

 nal body. This part of the medial wall is thick in lower forms 

 but the great arching of the corpus callosum in higher mammals 

 has stretched it out into the thin septum pellucidum. 

 * This very clear conception of the commissures and their rela- 

 tionships has been generally accepted and has been followed by 

 the writer. Elliot Smith and others have carried this concep- 

 tion into the study of reptiles and have based upon it some 

 homologies in the brains of amphibians and fishes. The writer 

 has approached all questions of brain morphology from a dif- 

 ferent standpoint. He has directed his attention always to the 

 more primitive forms, making the attempt first to see the struc- 

 ture and relationships existing in those brains, with the hope 

 that thereafter it would be possible to see with certainty how 

 the specialized brains of higher forms have been developed from 

 the lower. The method heretofore followed by most workers 

 has been to study man and mammals first, as the animals in 

 which we are directly interested, and then to apply the results 

 to lower vertebrates either as a matter of curiosity or as a means 

 of securing corroborative evidence of their accuracy. The ge- 

 netic method is more tedious but if followed with caution and 

 fidelity to the actual facts it should lead to permanent results. 

 Caution is necessary in the forms chosen for study and the 

 significance accorded to the facts discovered. If we are to use 

 lower forms to throw light upon the human brain, those forms 

 must be chosen for study which are at once the least specialized 

 and most nearly allied to the ancestors of mammals and man. 

 In studying such brains attention must be given to the habits 

 of the animals and the consequent degree of development of 

 each system of nerve centers. 



