Herrick, Callosimi and Hipocampus m Lower Brain. 177 



tions in adult higher mammals. The origin of the hippocam- 

 pus as a fold or convolution of the cortex is as clear as any such 

 process can possibly be. For the present, however, we deal 

 with a question of fact. Does a separate commissure exist in 

 the position to which the writer assigned it ? In this we may 

 be deceived and may fairly be asked to produce the evidence. 

 The series of horizontal sections of the brain of a pouch 

 embryo of Didelphys may first be examined. The relations of 

 the fornix and hippocampus are very clearly shown by this set 

 of drawings. The relation of the fimbria as a band of fibres 

 running along the ventricular aspect of the negative convolution 

 forming the "fornicate gyrus " is perfectly evident. The com 

 missure itself with its decussating elements forms in its trans- 

 verse section (i. e. in saggital sections of the brain) a horse-shoe 

 figure with the convexity dorsad. This is due to the cephalo- 

 caudal curvature of the hippocampal structures which is also a 

 dorsal curvature. Thus the middle portion of the commissure is 

 cut in the dorsal horizontal sections Figs. 4-5, Plate IX, while far- 

 ther ventrad the caudal and cephalic parts of the commissure ap- 

 pear. Fig. 6. The cephalic limb of the commissure might readily 

 be regarded as distinct, but, although the arch of these fibres is 

 cephalo-dorsad and that of the caudal portion is caudo-dorsad, 

 the fibres seem to terminate in the forward extension of the 

 same system. I shall not deny the possibility that these, or 

 some of them, really correspond to callosal elements but desire 

 to call attention to the existence of other fibres which pass 

 cephalad of the hippocampal system ; these are shown in Fig. 

 7. It will be seen that the position of these fibres, which are 

 separated by a slight interval from the hippocampal, strongly 

 reinforces the writer's position (primarily based on the results 

 of localization experiments) that the motor cortex is thrown far 

 cephalad. ' 



'One may be forgiven for a desire to homologize the transverse fissure of 

 the opossum cortex with the crucial sulcus of carnivora (if he refrains from 

 actually doing it!) It has been impracticable to repeat the rather unsatisfactory 

 localization experiments thus far, but the recent results in the comparative mor- 

 phology and psychogenesis of the sensory areas cetainlydoes not militate against 

 the view that a very large share of the mantle of the opossum is Sensory and that 

 upon this area the olfactory sensation has a relative disproportionate claim. 



