Fish, TJic hidusinni of tJie Callosum. 65 



gleich, welche die lateralen Wande desselben Ventrikcls, also 

 die Lanmiae Septi bedeckt. Audi enthalt sie markhaltige sag- 

 ittate Nervenfasern." If it be true that the triangular area that 

 is to form the hemiseptum remains circumscribed throughout 

 development (in man) by the callosum, fornix and copula, then 

 there would exist an effectual barrier to the intrusion of the cor- 

 tex into this region. In the absence of figures and a specific 

 description of the formation of this cortex, there is at least room 

 for a difference of opinion. In the sheep, that part of the sep- 

 tum immediately adjacent to the ventral surface of the callosum 

 presents no evidence of cerebral cortex. 



In the specimens examined for this paper no attempt was 

 made to go into the fine histological structure, but to determine 

 the morphological relations of this region in some animal forms 

 that have not, as yet perhaps, been studied in this connection. 



In the human indusium there is a distinct indentation of the 

 surface at the meson, with an elev^ation at either side gradually 

 sloping laterad toward the bottom of the callosal fissure and 

 near its point of union with the true cerebral cortex, becoming 

 so much thinned out in some regions as to be scarcely percept- 

 ible. At about the middle of the distance from the meson to 

 the bottom of the callosal fissure, there is a short isolated layer 

 of nerve cells lying nearer to the dorsal than to the ventral bor- 

 der. The cells are pyramidal. 



The highest level of the indusium was not found at the lay- 

 er of the cells, as is commonly shown. The indusium is com- 

 posed chiefly of an extension of the neuroglia layer of the cerebral 

 cortex, in which are also found true nerve cells, varying in num- 

 ber and arrangement. The " grain " of the indusium is more 

 or less vertical, while that of the callosum is distinctly transverse. 



The brain of the Chimpanzee, Troglodytes ?ngcr, had been 

 medisected so that the absolute continuity of the indusium 

 across the meson was not determined. It is, however, to be 

 strongly inferred since the cut was made a little laterad of the 

 meson and the true dorsal surface of the callosum was not ex- 

 posed. 



The layer of nerve cells differs from that of man in being 



