ORIGIN OF THE CORPUS CALLOSUM. 61 



Tims it icould appear that in this bat cominissural fibres arising from a very restricted 

 area of the dorso-mesial cortex above the supracommissural region of hippocayrtpus, 

 instead of passing by the circuitous route via the external capsule and through the 

 ventral commissure, pass through the alveus of the supracommissural hip)p)Ocampus to 

 reach the opposite hemisphere through the dorsal limb of the dorsal commisstire. In 

 other icords, fibres tohich do not </rise in the hippocampal formation jjass through the 

 alveus of the hippocampus to become commissural, and, as a result of the consequent atrophy 

 of the in faded region of hippocampris, the invading fibres ustirp the place previously 

 occupned by the anterior part of the hippoeamp)al commissure. 



The " usurping fibres " are the " corpus callosum, and the atrophic hippocampus 

 forms part of the " inclusium " and " striae Lancisii." 



The relation of the rudimentary corpus callosum to the hippocampus, to the " cora- 

 missure-bed," and to all the surrounding structures is identical in every respect with that 

 which the corresponding part of the hippocampal commissure presents in the Marsupial. 

 We have no reason to regard the " commissure-bed " in the bat in any other light than 

 as the strict homologue of the structure which we have designated by that name in the 

 bandicoot. For even though the large mass of hipjiocampal cortex is wanting upon the 

 dorsal aspect of the commissure, it cannot have been absorbed into and swell the mass of 

 the " commissure-bed," for its vestiges are clearly and indubitably present, as we have 

 just seen, upon the dorsal aspect of the " commissura dorsalis." Since our new 

 commissure or corpus callosum lies entirely upon the ventral aspect of the hippocampus, 

 it must be still contained in the " commissure-bed " (or possibly the " corpus prtc- 

 commissurale ") just as in the Marsupial. 



General Considerations. 



We have now contrasted the hippocampal region of a typical Marsupial cerebrum 

 with that of certain Cheiroptera as types of an extremely low grade of Eutherian 

 organization. I propose now to In-iefly discuss the significance of the differences observed, 

 and their special bearing upon the question of the origin of the corpus callosum. 



Fig. 1 (p. 63) represents in a schematic manner the position which tlie hippocampus 

 occupies in the mesial wall of the hemis})here of a Marsupial. This hippocampus may 

 be roughly divided into three segments: — {a) a "precommissural segment" [hipj"), 

 which lies in front of the commissures and sejiarates the precommissural area [p.a.) 

 from the general cortex {p) ; {b) a " supracommissural segment " {hip'), which lies 

 above the dorsal commissure, whose dorsal limb separates it from the " commissure- 

 bed " (s) ; and (c?) a " postcommissural segment," which constitutes the greater part of 

 the hippocampal formation and possesses a true fimbria [fi). 



These three segments of the hippocampus form an extensive arc, reaching from the 

 olfactory bulb {o.b.) in front to the lip of the temporal pole {t) behind and below. The 

 " commissura dorsalis " derives its fibres from this extensive arc. [The few fibres 

 springing from the precommissural body \\ hich probably enter into the constitution of 

 the dorsal commissure may be disregarded in this discussion.] The commissural fibres 



