330 Robert J. Terry. 



ported by mesenchyma rich in blood-vessels. The latter, contin- 

 uous with vascular channels in the vicinity of the epiphysis, post- 

 velar arch and paraphysis, are close under the velar epithelium. 

 No difference was observed in the thickness of the epithelial coat 

 of the anterior and posterior aspects of the velum as has been de- 

 scribed in Acanthias by Minot ('01) and in Acipenser by Kupffer 



C06). 



Post-velar Arch (Fig. 2, PV). The name ''post-velar arch" 

 was given by Minot ('01) to the curve in the brain roof which lies 

 between the velum and the epiphyseal anlage. This is the part 

 of the diencephalic roof called Zirbelpolster by Burckhardt and 

 the region of the post-paraphysis of Sorensen ('94). Out of this 

 region is developed in the ganoids and some of the bony fishes 

 the evagination called by Goronowitsch ('88) the dorsal sac. In 

 Ospanus, the arch does not expand into a sac, but on the contrary 

 diminishes in extent and finally disappears. In the stage under 

 discussion, the post- velar arch is a small dome-shaped evagination 

 of the diencephalic roof extending sagittally between the superior 

 commissure and velum and going over laterally into the intermedi- 

 ate tubercles. At no time is the evagination so extensive as it is in 

 Salmo where, as Hill ('94) states (p. 242) ''it bears some resem- 

 blance in form to the epiphysis." The arch in Opsanus rises 

 a little higher than the base of the epiphysis but not so high 

 as the paraphyseal fold. Its simple cavity can be followed for- 

 ward to the pedicle of the velum, on either side of which it becomes 

 continuous with a short blind recess (fig. 6). In transverse sec- 

 tions through the posterior part of the velum, these two small 

 recesses appear as lacunae surrounded by the velar and ependymal 

 epithelium. In structure the post-velar arch consists of a layer 

 of rather thick ependyma. Anteriorly, this continues witli slight 

 increase in the height of its constituent cells into the velar epi- 

 thelium. Posteriorly, it changes .abruptly to a very thin layer at 

 the level of the superior commissure. The latter lies upon the 

 epithelium of the caudal part of the arch, over which it extends 

 in a lateral direction as far as the intermediate tubercles. Folds 

 of the epithelium are not present, nor are there any diverticula 

 such as Herrick ('91) has found in the walls of the dorsal sac of 



