646 CEREBRAL COMMISSURES OF THE MAMMALIA, 



limb of the hippocampal commissure does in Marsupials, and 

 develops in the lamina terminalis just as the corpus callosum does 

 (Marchand, Paul Martin, Minot). Moreover the latter is con- 

 tinuous posteriorly with the psalterium. The corpus callosum 

 from comparative reasons, therefore, as well as from the purely 

 ontogenetic evidence in human brain by Marchand,* and Paul 

 Martin in the cat,t lies entirely ven^ra^ to the Randbogen. Hill's 

 statement! that " the corpus callosum in its backward extension 

 breaks through a convolution, which lies outside the ring from 

 which the fascia dentata, fimbria and fornix are developed," as 

 well as the opinion of Fish " that in its dorso-caudal growth the 

 callosum ploughs its way . . . through the arcuate gyre "§ 

 are as equally opposed to the facts of comparative anatomy as 

 they are quite unsupported by the brilliant results of Marchand's 

 developmental researches. 



Before a corpus callosum (using the term in the restricted sense 

 in which anatomists generally regard it, i.e., as distinct from the 

 psalterium) can develop, either the hippocampus must disappear 

 in the region in which the supraventricular callosal mantle-com- 

 missure is to develop, or the callosal fibres must traverse the 

 hippocampal region. Of these alternatives the former is that 

 which is found to take place. Accordingly in the ontogeny of 

 the cerebrum of the placental mammal the anterior portion of the 

 fissura arcuata (the "vordere Bogenfurche" of His), and con- 

 sequently the hippocampal projection into the anterior horn and 

 body of the lateral ventricle, divsappears preparatory to the 

 development of the corpus callosum, the " hintere Bogenfurche " 

 alone persisting to become the hippocampal fissure, which forms a 

 projection limited to the descending horn of the ventricle. This 

 is quite in accordance with the belief of Professor D. J. Cunning- 

 ham, who, as the result of his study of the development of the 



* " Ueber d. Entwick. d. Balkens im Menschlichen Gehirus," Arch, f, 

 Mikr, Anatomie, Bd. xxxvii. 1891. 



t Loc. cit. 

 X " Tlie Hippocampus," Phil. Trans. B. 1893. 

 S " The Indusiuui of the Callosum," Journ. of Conip. Neurology, Vol. iii, 

 1893, p. 61. 



