The Pineal Region in Teleosts. 351 



■"O 



of this fiber bundle immediately in front of the epiphysis does not 

 always obtain, as Cameron points out, and a definition of its 

 relations applicable to all cases must wait until more observations 

 have been made. One relation appears to be constant, namely, 

 that the commissure is associated with the post-velar arch. This 

 relationship is shown in median sagittal sections of the brain 

 where it appears that the diencephalic roof in front of the 

 epiphysis is composed of two strata, the cellular ependyma and the 

 narrow fiber layer of the superior commissure. When these are 

 followed in a direction away from the median plane they are found 

 to pass over to the intermediate tubercles. Here all three of the 

 fundamental layers of the brain wall, as recognized by His ('89) 

 and Minot ('92, '03) can be seen, the Randschleier or ectoglia 

 layer being formed by the fibers of the superior commissure. The 

 latter may therefore be regarded as the ectoglia layer of the post- 

 velar arch, small and limited to the posterior aspect of this region 

 in Opsanus, but more extensive in Amia and Petromyzon. The 

 origin and early disposition of the posterior commissure would 

 warrant the same conclusion respecting its relation in the brain 

 wall. 



Post-velar Arch 



The post-velar arch reaches its highest development in Opsanus 

 when the embryos are about 8 mm. long, and subsequently dis- 

 appears, probably by incorporation with the velum. The epi- 

 thelium of the arch is like that of the median lobe of the velum 

 and probably functions as a secreting surface. Hill ('94) and Ley- 

 dig ('96) have found that this part of the brain-roof in teleosts is 

 specially differentiated to form ridges and secondary folds of 

 the epithelium resting upon a connective tissue foundation, in 

 some cases vascular. Leydig states that in the trout the cells 

 are higher toward the summit of the arch. The great expansion 

 of the post-velar region in the ganoids is well known through the 

 writings of Balfour, Huxley, Wiedersheim, Goronowitsch, Wilder 

 and others in recent years. In Amia and Lepidosteus, Kingsbury 

 ('97) found the dorsal sac, velum and metaplexus lined with an 



