58 DR. G. ELLIOT SMITH ON THE 



If we examine a series of transverse and sagittal sections of a reptilian brain, it will 

 be found that both ventral and dorsal commissni-es lie in the easily recognizable lamina 

 terminalis. This lamina contains in the median j)lane a minimal amount of grey matter. 

 But on either side of the median plane this thin bridge of grey matter becomes 

 continuous with a huge mass of grey matter which we readily recognize as the 

 backward continuation of the " precommissural body." Thus in the Reptile the 

 " commissure-bed " consists of the posterior extremities of the " precommissural arese " 

 united by the thin grey lamina formed by the lamina terminalis. 



In the mammal it is evident that either the grey mass of the " precommissural area" 

 has invaded, and thus thickened, the lamiua terminalis, which liecomes a continuous grey 

 mass across the mesial plane, or, on the other hand, the mesial surfaces of the two 

 posterior regions of the precommissural area may have become " glued " together. One 

 or other event must have occurred ; and the evidence, I believe, points to a gradual 

 thickening of the lamina terminalis by the invasion of grey matter from the adjacent 

 " precommissural area." However the thickening is brought about, we know that the 

 two cei'ebral commissures of the Marsupial lie in the " commissure-bed," which is 

 morphologically a thickening of the lamina terminalis. Yet we are equally sure, both 

 from its topographical relations to surrounding structures and from its minute structure, 

 that the " commissure-bed " is morphologically part of the same sheet of grey matter as 

 the " precommissural area." 



If the hippocampal region be studied where it bulges in the descending limb of the 

 lateral ventricle in any Eutherian brain, a considerable variety of the form and relative 

 sizes of its constituent parts will be apparent. In the Marsupial brain equally great 

 variations are found in the corresponding region of the hipj)ocampus. But if we 

 examine the supracommissural part of the hippocampus in a series of Marsupials, the 

 greatest variety and most varied degrees of complexity are demonstrated. In Notoryctes, 

 and less markedly in Dcmjiimis, the formation is very simj)le. In Ilacropus it is much 

 more complex. Perameles is an example of a highly-developed type. Both Monotremes 

 present in the supracommissural region a more complicated form of hippocampus 

 than is fovmd in any other mammal. But the anterior extremity of the hippocampus 

 undergoes the same process of unfolding and simplification in all Marsupials and 

 Monotremes which I have so briefly described in Ferameles. Hence, should we find a 

 difference in the relative degree of development of various parts of the hippocampal 

 formation in the brains of two different animals, we must not lay much weight upon 

 such differences as an indication of the systemic position of their possessor. 



The Mippocampal Region in the J3at. 



The cerebrum of the bat may now be studied in the same manner as that of the bandi- 

 coot. Beginning with a coronal section through the hippocampal region of Miniopteriis 

 immediately behind the region of tlie commissures, one cannot fail to notice the great 

 similarity between the two forms (figs. 12 and 1). What differences do occur, apart 

 from those of al)solute size, point to a less complex form of structure in Miniopterus 

 (fig. 12) than in Perameles (fig. 1). This simpfication consists of a diminished depth of 



