OEIGIN OF THE COEPUS CALLOSUM. 67 



of an adult man. The anterior extremity of the corpus callosum has become so bent 

 that its rostrum (r.) becomes continuous with the remains of the " commissure- 

 bed," which still surrounds the ventral commissure (c.v.). In this way the septum 

 pcUucidum {sept.) becomes surrounded by a complete ring of the matrix of the 

 commissiu-es and cut off from the gyrus subcallosus, which is the remainder of the 

 precommissural area. This gyrus subcallosus extends down to the base of the cerebrum 

 immediately in front of the ventral commissure and lamina cinerea {I.e.). Its line 

 of separation from the pallium is indicated in the figure by a dotted line {a.l.). 

 The gyrus subcallosus is traversed by a well-marked vertical depression — the Jissm-a 

 jirima {fiss.])!'.), — Avhich has no important morphological significance. 



With the knowledge which we have acquired of the process of phylogenetic 

 development of the region of the commissures, it is extremely instructive to study 

 the beautiful figures with which Marchand, in man, and Paul Martin, in the cat, 

 represent {op. cit.) the process of ontogenetic development. 



In the Marsupial we have a fissura arciiata or hippocampi, extending from the 

 tip of the temporal pole right round the mesial wall of the hemisphere towards the 

 olfactory pedimcle ; so, in the foetal child or kitten, we find the Bogenf urche (wliich we 

 might, with Mihalkovics, appropriately call " Ammonsfurche ") following a similar course 

 and shading away towards the cephalic pole of the hemisphere. And it is necessary to 

 remark, in passing, that the so-called part of the " vordere Bogenfurche," which His 

 calls " fissui'a j)rima," has noticing whatever to do with the true Bogenfurche or 

 fissura arcuata, if we regard the latter as the primitive fissura hippocampi. 



In the early stages of the cat, the lamina terminalis becomes thickened and invaded 

 by the commissural fibres of the fornix (Paul Martin), so in phylogcny we have the 

 corresponding stages in the adult Monotremes and Marsupials. 



Then, as the commissural fibres increase in number, the grey mass or thickening of 

 the lamina terminalis is invaded by so many white fibres that the grey substance seems 

 to disappear, but it is in reality being gradually extended by the swelling commissure. 



The backwardly-extending commissure produces exactly the same series of changes — 

 the same hippocampal flexure — in our phylogenetic series as it does in the developing 

 brain of the cat and man (see Martin and Marchand's figures). 



Thus, for oil the stages in the developing brain of the cat, we can find almost exact 

 prototypes among the more lowly-developed mammals. 



The great feature which far more than any other distinguishes the mammalian brain 

 from that of all submammalia is the possession of a definite pallium — -distinct alike 

 in its histological features and its morphological relations — giving rise to a definite 

 internal capsule of projection-fibres and well-defined and fully-medvillated commissural 

 fibres. At first, in the Monotremata and Marsuj)ialia, this pallium (like the parent mass 

 of the basal ganglion from which it ajipears to have sprung) is united to its homologue 

 of the opposite hemisphere by means of the " commissura ventralis " — " the commissure 

 of the cerebral hemisphere " par excellence. 



But the rapid growth in extent and complexity of this general cortex or pallium 

 is accompanied by a richer and more abundant commissural system. This growing 

 commissural system from the dorsal part of the enormous pallium not only finds in the 



