CEEEBEAL COIMMISSUEES IX THE TERTEBKATA. 495 



But in the light of the above recorded observations upon tlie contmissuru abcrrat/s, it 

 seems more than probable that these hemispheral fibres of the commissura superior in 

 the Tchthyopsida represent the aberrant hippocampal comiiiissurc of the Lacertilia and 

 Rhynchocephalia, and do not strictly belong to the habenular or superior commissure. 

 This interesting fact explains the otherwise inexplicable confusion of commissura 

 superior and coMmissura aherrans in the mind of Osl)orn (^lorph. Jahrb., Bd. xii. pt. i.). 



Up to this point I have altogether refrained from speaking of the conditions which 

 prevail in the Ichthyoj)sida, because the crucial problems witli whicli this essay is more 

 especially concerned find their readiest solution in tlio study of the Amniota. For, once 

 a trustworthy Imsis for comparison of the Sauropsida and Mammalia is established, and 

 the factors which are at play in the specialization of the various types, which Reptiles, 

 Birds, and I\[ammals afford, are appreciated, the key to the solution of the problems of 

 Ichythopsidau morphology is provided. The comparison of a simple reptilian Ijrain, such 

 as the Cheioaian type with the Amphibian, is a much simpler and more obvious process 

 than the comparison of the reptilian brain with that, say, of a placental mammal. 



In the Amphibia all gradations of brain-form are found, from a type which is 

 practically identical with that of Protopterus or Lepidosireii, and differs relatively 

 slightly from that of Ammocwfes, up to a form which readily admits of comparison 

 Avith the Chelonian type. 



It is quite lumccessary to enter here into any long account o£ the features of the Ichthyopsidan types 

 of cerebral hemisphere. So mueli has been written upon this subject within recent years, and the 

 present state of knowledge has so recently been succinctly stated by Gaupp (' Auat. d. Frosches/ 1897) 

 and Johnston (Journ. Coinp. Neurol. 1900, 1901, and 1902) among many others, that the general facts 

 concerning these types are now matters of common knowledge. But the question of the interpretation 

 of this huge collection of facts is a very different matter, and the utmost confusion reigns in the field of 

 Ichthyopsidan neurology. It will be sufficient, therefore, if I indicate, in the light of the conclusions 

 drawn from the comparison of Sauropsida aud Mammalia, the main sources of fallacy in the current 

 interpretations of the Ichthyopsidan cerebral hemisphere, aud point out what seems to me the true 

 basis for accurate comparison with higher forms. 



In the history of the growth of anatomical science, more especially as regards the Yertebrata, it so 

 happens that most of the names of the various organs and of the subsidiary parts of the body were 

 originally bestowed upon the human subject : later on these names have been applied with more or less 

 accuracy to those parts in other animals which correspond, or are supposed to correspond, to tiie 

 structures so-named in the human body. In the case of the brain, for example, comparisons were 

 naturally made with the corresponding organ in other Mammals, and applied with doubtful results to 

 Keptiles and Birds. In the case of the Amphibia, the data which were available for the accurate 

 application of the terms of Human Anatomy have of necessity filtered through the uncertain channels 

 of the lowlier Mammalian and Sauropsidan neurological knowledge, and in the process have become 

 tainted and vitiated with all the inaccuracies which I have already ex|)osed in speaking of the reptilian 

 brain. Added to all the inaccuracy which necessarily results from such a process, there is the further 

 fertile source of error which results from futile attempts at the comparison of the Reptile and Amphibian. 

 Under such circumstances, it is no matter for surprise to find the present state of knowledge concerning 

 the morphology of the brain in the Amphibia almost chaotic. 



One example may suffice to show that this latter remark is no mere figure of speech iu regard to 

 recent work in this particular field of research. 



SECOND SERIES.— ZOOLOGr, VOL. VIII. 72 



