328 Robert J. Terry. 



shows the superior commissure (S) lying immediately anterior 

 to and in contact with the base of the epiphysis. It occupies a 

 position in the outer part of the diencephalic roof, resting ventrally 

 upon a thin ependymal layer (Comp. Acanthias, 22 mm., Minot, 

 '01). The component fibers, non-medullated at this period, pass 

 over the surface of the post-velar arch to the intermediate tubercles. 



Posterior Commissure. The posterior commissure is located in 

 that fold of the brain-roof which is recognized as the boundary 

 between the mid-brain and fore-brain. Sagittal sections show it 

 to be folded transversely. The two layers resulting from this 

 disposition of the commissure are continuous at their ventral edges. 

 They extend from side to side and lie, the one dorso-caudad of the 

 other (fig. 2, PC). Rabl-Ruckhard ('82), Haller ('98), Mayser 

 ('82), and other writers, have noted these two divisions in teleosts. 

 Haller has described the commissure in Salmo as composed of a 

 dorsal and a ventral part, the former belonging exclusively to the 

 lobi optici and carrying fibers from lobe to lobe, the latter made up 

 of mixed fibers of the longitudinal tracts. In the toad-fish, a sep- 

 tum of mesenchyma, continuous with the same tissue around the 

 epiphysis and mid-brain, separates the two layers for a considerable 

 distance. The ventral or anterior stratum, an even layer of fibers, 

 is separated from the third ventricle byathick ependyma. It 

 reaches forward as far as the epiphysis, occupying that region of 

 the diencephalic roof called the Schaltstuck or pars intercalaris 

 (Burckhardt) . Some of the fibers pass around the base of the epi- 

 physeal stalk. The dorsal or posterior stratum is related to the 

 wall of the mid-brain. Its thickness varies inversely with that of 

 the underlying epithelium in such manner that, whereas the 

 ectal surface of the stratum is even, the deep surface is irregular. 

 When followed in the dorsal direction, this stratum merges with 

 the. ectoglia layer of the mid-brain roof. 



Velum transversum. (Fig. 1, V). This structure, projecting 

 from the roof of the brain into the ventricle some distance in front 

 of the superior commissure, is naturally divisible into two parts. 

 One of these is a low, broad, transverse fold of the roof, the other 

 a large ovoid body, hanging from the middle of the fold by a short 

 pedicle (fig. 2, V). In the embryos of teleosts, ganoids, and elas- 



