77*!^ Brain of Diemydylus Viridescens 263 



tiles. This he calls Amnion's horn as it connects with 

 olfactory nerve fibers and represents the cortical olfactory cen- 

 ter of higher forms. Whether a similar connection and func- 

 tion can be demonstrated in diemj^ctylus is not known. 



The paracoeles open by wide portse into the aula (Fig. 17, 

 36, 37). The cavity of the latter is nearly filled by a plexus, 

 auliplexus (p. 265), so that the portse are not visible from the 

 meson (Fig. 6). 



The caudal limit of the aula is defined by a band of alba or 

 white matter rising from the floor of the brain, in the terma. 

 This is formed by the cerebral commissures. The more dorsal 

 portion is the callosum (38, 42), which in the form of a horse- 

 shoe sends lateral columns dorsad (Fig. 6, 18, 19, 42, 51) into 

 the mesal walls of the hemicerebrums (Fig. 35-37)- 



These mesal walls of the hemicerebrums as they bulge into the 

 cavity of the paracoeles have sometimes been called striatums, 

 in amphibia. Osborn (38, Fig. 9) shows the fibers of the 

 callosum distributed in this region of the frog to ' ' the upper 

 median cell area," while in reptiles and birds he found in this 

 region a "sulcus" or fissure, the hippocampal. Nakagawa 

 (35) considers the cells of this region in spelerpes to be a rudi- 

 mentary cortex. In diemyctylus this region is clearly defined, 

 contains numerous, but well separated cells in large peri- 

 cellular spaces (Fig. 41, ce?), extends from the rhinocoele to 

 the portae (Fig. 36) and caudad over the portse to near the tip 

 of the cerebrum (Fig. 21, 35). The fibers of the callosum 

 spread out between these cells. There is no indication of a fis- 

 sure but it seems proper to use the term callosal eminence to in- 

 dicate this ridge pushing into the paracoele, as it corresponds 

 in position to that eminence as shown by Wilder (55, Fig, 

 4748) in a human foetus. 



In figures 17-19, 51, angles in the paracoeles are seen asso- 

 ciated with projections of cinerea which form two horns curving 

 outward and toward each other. This enclosed, lateral region 

 of the cerebrum, through which the fibers of the precommissure 

 pass, may be considered as a very undeveloped form oi striatum 

 — inasmuch as in other forms this region has been so homol- 

 ogized. 



