66 Journal of Comparative Neurology. 



nearer to the ventral than to the dorsal margin. The cells are 

 also relatively more numerous. At the mesal end of the layer 

 of cells, and more dorsal than ventral, there is an irregular area 

 of loose texture which corresponds, perhaps, to the longitudi. 

 nal bundle of fibers. 



In a monkey, Macacus cynoinolgus, the most striking fea- 

 ture of this region is the absence of any indication of a callosal 

 fissure and the close approximation of the hemicerebrums. At 

 the bottom of the intercerebral fissure lies the single large cal- 

 losal artery, separated trom the callosum by an exceedingly thin 

 and small indusium. No nerve cells are present and but few 

 neuroglia cells. 



The sheep presents an almost typical arrangement for this 

 region, justifying the diagrammatic representation of it on PI. 

 XXVI. of Wilder's "Physiology Practicums." 



The indusium as a whole is very distinct and can readily be 

 separated into a superficial and deep layer. The nerve cells are 

 almost directly continuous with the layer of pyramidal cells in 

 the true cortex. It becomes very much thinner at the meson 

 and, as in the other forms, is continuous. The nerve cells do 

 not appear in this thin portion. In a Golgi preparation the 

 fibers from the cells run almost directly transversely. 



The indusium of the cat differs from any of the other forms 

 studied, in being non-continuous across the meson. The section 

 was taken from the region of the splenium and it is quite proba- 

 ble that the break in continuity is not as great farther cephalad 

 as it is here. The nerve cells continuing from the cortex to the 

 mesal edge of the indusium, fig. 6. 



This want of continuity is not easily explained unless we 

 may consider that, during development, there has been so great 

 a growth in a lateral direction as to rupture this delicate cover- 

 ing and to leave quite an area of the dorsal surface of the cal- 

 losum, along the meson, bare. 



The morphological significance, then, of the indusium is 

 the retention in the adult, of a feature that is quite prominent 

 in the embryo. It is important also in that it brings the cal- 

 losal fissure into line with all the other true cerebral fissures in 



