BY G. ELLIOT SMITH. 649 



improbable that in the 8auropsida and Amphiliia such a com- 

 missure should exist, as so many writers assert. 



Although a jyriori statements are of little value as scientific 

 argument, still with such presumptive evidence of the absence of 

 a corpus callosum in Submammalia, one is naturally chary of 

 accejiting any opinion as to its presence, unless supported by 

 definite and convincing evidence. Such evidence moreover is not yet 

 forthcoming, and the arguments which have hitherto been adduced 

 in support of the hypothesis, are of the flimsiest kind. Herrick 

 states* "the callosum is practically absent [in Didelphys\ 

 a rudiment of what may be called corpus callosum [is present], 

 although we are unwilling to definitely homologise it with that 

 body ... it soon loses itself in the median walls of the 

 hemisphere, corresponding to the septum pellucidum. Being a 

 tract of cortex, this band has as great claim to be homologised 

 with the corpus callosum as the relatively larger commissure of 

 the alligator." The area which Herrick here calls "septum 

 pellucidum" and elsewhere "intraventricular lobe" is an area of 

 the mesial surface of the cortex, which is directly continuous 

 posteriorly with the tract which has already been described as 

 the septum lucidum and which develops from the lamina termi- 

 nalis. The corresponding grey mass in Reptiles has been called 

 " septum pellucidum " and " Fornix leiste " by Edingerf and 

 Meyer,! although the latter would not definitely homologise it 

 with the " septum" of Eutheria. Flower§ called it " septal area." 

 All these terms are very misleading, because the area in question 

 is not homologous with the septum lucidum, which is mex'ely the 

 thickened lamina terminalis.|| This tract, moreover, develops 

 from the "posterior olfactory lobule" of His, and can be I'eadily 



* "Cerebrum and Olfaotories of the Opossum, &c.," Bull.Sci. Laboratories, 

 Denison University, Vol. vi. Part ii. p. 75. 



t " Vergleichend-entwick. und anat. Studien im Bereiche der Hirn- 

 anatomie. 3. Riechapparat und Amnionshorn,'' Anat. Anzeiger viii. 10, 11. 

 X " Ueber das Vorderhirn einiger Reptilian," Zeit. f. wiss. Zool. \v. 

 Bd. Nov. 1892. 



§ Loc. cit. 

 ii Minot " Human Embyrology." 

 44 



