42 'Journal of Comparative Neurology and Psychology. 



which was only poorly explained by the investigations of Viault, 

 RoHON and Sanders. 



The macroscopical structure of the so-called peduncular part 

 of the 'tween-brain, also called prcethalamus, situated between the 

 fore-brain and the place where the optic nerve enters ventrally and 

 the ganglia habenulas dorsally, resembles very much that of the 

 teleosts. Both show dorsally a plicated ependymal membrane 

 and ventrally a small base of gray material and laterally the walls 

 thickened by the fore-brain tracts which pass through them. The 

 roof of the mid-brain overlaps the 'tween-brain less than in the 

 teleosts and does not touch the ganglia habenulse laterally. The 

 lobi inferiores, whose saccus vasculosus is more developed in the 

 selachians than in the teleosts, extend caudad farther than the 

 superficial origin of the oculomotorius (Figs Ixvto Ixix, Plate IV). 



The tracts of the praethalamus are partly the same and partly 

 different from those of the teleosts (see Fig. 6, Plate XIII). For 

 the latter fishes I was able to mention as fore-brain attachments: 

 (l) tr. strio-thalamicus; (2 and 3) tr. olfacto-lobaris medialis et 

 lateralis, and (4) tr. olfacto-habenularis. For the selachians there 

 are to be mentioned: (i) tr. strio-thalamicus; (2) a part of the tr. 

 medianus running along with the former in the back part of the 

 fore-brain; (3) tr. pallii and (4) tr. olfacto-habenularis. 



That the tr. strio-thalamicus is homologous in both fish types 

 is evident on account of the fact that it begins and ends in the 

 same place in the selachians as in the teleosts. 



Whether one may consider the two tr. olfacto-lobares of the 

 teleosts which begin in the secondary olfactory regions and end in 

 the lobi inferiores as homologous w^ith the tr. medianus and tr. 

 pallii, which also originate in the secondary olfactory regions of 

 the fore-brain, is a question which I may not answer before having 

 described their further course. 



I saw a small part of the terminal fibers of the tr. olfacto- 

 habenularis, which is situated directly inward from the tr. pallii, 

 pass over into a small homolateral group of cells belonging to the 

 area of the ganglia habenulae (Fig. liv, Plate III), but situated 

 somewhat lateral and ventral to the chief group of the anterior 

 ganglion. I have formerly mentioned this group, the cells of 

 which are distinguished by a polygonal shape from the more 

 round cells of the true ganglia, and called it the "ganglion inter- 

 calatum tracti olfacto-habenularis." It is certain, however, that 



