654 CEREBRAL COMxMISSURES OF THE MAMMALIA, 



teriorly into the fascia dentata, are connected antei-iorly with the 

 olfactory lobe, in much the same way as I have elsewhere shown 

 my '■'■ oljactory bundle of the fascia dentata" to be connected in 

 Platypus. 



After the appearance of the hippocampal commissure the 

 increased development of the mantle next demands a shorter 

 course for its connecting fibres, which is provided by the corpus 

 callosum. If these conjectures are right, the three cerebral com- 

 missures must be regarded as serially homologous, the anterior 

 commissure, or at least its pars olfactoria,* appearing first, the 

 hippocampal commissure next, and the corpus callosum last. 

 That some of the fibres, which in Placentalia pass in the latter, 

 cross the middle line with the anterior commissure in Non- 

 Placentalia seems certain, so that part of the anterior commissure 

 is really the homologue of the corpus callosum in the Proto- and 

 Metatheria, the hippocampal commissure being merely a part of 

 the anterior commissure, which has become separated off earlier in 

 phylogeny. The appearance of a corpus callosum can hardly be 

 considered then, therefore, such a sudden event as would appear at 

 first sight and such as Professor Huxley's remark (supra) seems 

 to imply, but is m'erely an example of the adaptive element in 

 development, which recent research has recognised as the important 

 modifying factor of phylogenetic tendencies in ontogeny. 



What becomes of the anterior extremity of the hippocampus 

 in the placental mammal 1 Comparison with the non-placental 

 mammal supports the view, which is forced upon one by the 

 important researches of Mai'chandf and Blumenau,J that the 

 gyrus supracallosus of Zuckerkandl§ (the indusiuvi or strim 

 Lancisii) is the representative of the anterior part of the 

 hippocampus. 



* The "pallial" partof the anterior commissure probably does not appear 

 until after the hippocampal commissure. 



t Loc. cit. 

 J " Zur Entwickl, v. feineren Anatomie des Hirnbalkens," Archiv f. 

 Mikr. Anatomie, Bd. xxxvii. 1891. 



§ Loc. cit. 



