Fish, The Indusimn of the Callosiim. 63 



row relatively at some distance from the margin of the hemicere- 

 brum, but approximately parallel with it. This is known as the 

 arcuate fissure or " Bogenfurche;" it involves the whole thickness 

 of the wall (total fissure) and ultimately becomes the callosal 

 and hippocampal fissures, the latter remaining a total fissure. 

 The portion of the embryonic brain substance situated between 

 the arcuate fissure and the encephalic cavity is the arcuate gyre 

 or "Randbogen." As the proton develops it assumes a more 

 or less triangular form, the apex pointing dorso-caudad. The 

 dorsal side represents the callosum the ventral the fornix 

 and the area between the septum (or hemiseptum). 



Relatively the precommissure seems to retain the approxi- 

 mate original position of the proton, while the callosum and 

 fornix grow away from it. During the process of growth the 

 side opposite the point of union of the callosum and fornix, or 

 the base of the triangle, seems to separate from its attachment 

 near the precommissure in the great majority of mammals, so 

 that these parts then present, roughly at least, an interrogation 

 point or falciform outline. This condition is also usually repre- 

 sented as occurring in man, but here is a morphological reason 

 for doubting it, since in the human brain before birth there 

 exists — and it remains throughout life — a direct continuation of 

 the rostrum of the callosum with the terma, the copula, and it 

 seems quite reasonable to consider this continuity of the parts as 

 having existed from the primitive stages. The alternative to 

 this postulate is, that these parts, originally one, have separated 

 and grown together again secondarily. It would be exceedingly 

 instructive to know when and how this occurs. 



In its dorso-caudal growth the proton ploughs its way, as it 

 were, through the arcuate gyres as far as the hippocampal re- 

 gion. As it grows the parts become more and more differen- 

 tiated and the arcuate fissures and gyres become less apparent. 

 It is the remnant of these gyres that forms the indusium of the 

 adult, and there seems to be ground for Blumenau's generaliza- 

 tion that the whole ectal surface of callosum is covered with a 

 layer of cortex of varying thickness. 



The indusium arches around the splenium and on its ven- 



