No. I.] DESMOGNATHUS FUSCA. 269 



caudal end of the hemicerebrums. This commissure as seen 

 in a medisection is quite high, projecting some little distance 

 into the cavity from the apparent floor, Figs. 3, 13. The 

 basal portion is partially made up by fibers from the basal 

 prosencephalic tracts. 



The callostim is separated from the dorsal portion of the 

 precommissure by a single layer of cells. The fibers of the 

 callosum arch dorsad like the letter U, Fig. 28, to help form 

 the roof of the paracoeles. They also bend cephalad in the 

 roof, Figs. 12, 13, and may extend in a latero-caudal direction, 

 while some, as has been noted by Osborn (38), seem to enter 

 the diencephal or to mingle with the fibers of the supra- 

 commissure. 



The supraco7nmissiire is well developed in the Des^nognatlms 

 and is in close connection with the habenae. It takes a ventro- 

 cephalic course, sending some of its fibers into the callosal 

 eminences or mesal walls of the hemicerebrums, A smaller 

 bundle remains in the diencephal, while a still smaller propor- 

 tion of the fibers arch around into the latero-caudal ends of the 

 cerebrum, Fig. 5. Osborn and Bellonci consider that both 

 commissural and decussation fibers are represented in this 

 commissure, while Herrick believes that two distinct com- 

 missures should be recognized, instead of one dividing into 

 two branches. 



Meynert's bundle {fascictilus retroflextis, Edinger). This is 

 a small but distinct bundle arising at the habenae. It takes a 

 latero-caudo-ventral direction and approaches the meson in the 

 floor of the mesencephal in the region of the interpeduncular 

 nidus. Osborn has found fibers running beyond this point in 

 Amphibia, while Ahlborn states that he has traced them into 

 the metencephal in Petromyzon. 



T\\Q postcommissiire is composed of myelinic fibers. It lies 

 caudad of the epiphysis and at a little distance from the surface. 

 Beginning at the postcommissure the fibers of the connnissm-a 

 tecti optici or sylvian commissure extend along the roof of the 

 mesencephal as a thin transverse layer. The course of the 

 postcommissure is more directly ventral than Meynert's bundle ; 

 its fibers soon become diffuse and I was unable to establish 



