No. I.] DESMOGNATHUS FUSCA. 247 



the intercerebral fissure ; the bulkier and principal portion 

 forms the floor of the mesal cavity from the chiasma to where 

 the aula sends off the paracoeles, and gradually merges into the 

 tela. It includes the precommissure and the callosum. 



The question as to the existence of a callosum in the lower 

 vertebrates is a very perplexing one, and perhaps as yet has not 

 been conclusively answered. In establishing homologies it 

 seems to me it is well to keep in mind not only the facts of 

 relation and distribution as they are more or less clearly shown 

 in the higher forms, but to consider the development of the 

 various stages and as far as possible correlate phylogeny with 

 ontogeny. Osborn (36) has conducted a most careful and 

 searching phylogenetic investigation of this subject and con- 

 cludes that the callosum exists in the various vertebrate classes 

 from " fishes " up. Symington (54) has investigated this region 

 in the marsupials and monotremes and in these low mammalian 

 forms finds the precommissure strongly developed and a hip- 

 pocampal commissure which has ordinarily been taken for the 

 callosum. In the absence of this commissure (callosum) his 

 conclusions concur with the earlier opinions of Owen, and are 

 opposed to those of Flower and some others. 



Since the callosum reaches its highest development in man 

 and its size is doubtless correlated with that of the cerebral 

 hemispheres it is, therefore, only proper that the conditions 

 found here where the morphology is clear and unmistakable 

 should be taken as the basis for homology. In man it forms 

 nearly or entirely the roof of the paracoeles or lateral ventricles. 

 It is composed largely of commissural association fibers joining 

 different areas of the two hemicerebrums, and contains like- 

 wise numerous collaterals from the cortical projection fibers. 

 It is unquestionably the fact that, in some of the lower forms, 

 notably the Amphibia, the so called callosum is, instead of a 

 roof of a cavity, apparently a portion of the floor of one — the 

 aula or ventriculus communis, and instead of being in front 

 and above (cephalo-dorsad) of the porta (foramen of Monro) it 

 is in some forms under and behind it (ventro-caudad). Some 

 of the more recent methods, especially Golgi's silver nitrate 

 method, do not seem to have been very extensively employed in 



