PARAPHYSIS AND PINEAL REGION IN REPTILIA 351 



ing more or less of a sinusoidal circulation. This complex in sec- 

 tions looks somewhat like a plexus, but when modeled it can be 

 clearly seen that the organ is absolutely distinct from any plexus. 

 Its wall consists of a single layer of cells as is the case with the 

 plexus, but its cells are always larger than the epithelial cells of 

 the plexus, velum and post velar arch. 



Velum and post velar arch 



The velum has been recognized in practically all classes of ver- 

 tebrates. It makes its first appearance as a slight infolding or 

 angle in the roof of the fore brain, which grows downward into 

 the brain cavity to a greater or lesser extent, and has a cephalic 

 and caudal wall with connective tissue between. It is continued 

 laterally into an internal ridge corresponding to an external groove, 

 which ends in the floor just behind the optic stalk at the optic 

 chiasma, figs. 2, 3, 5, 7, 8, and 17-19. As development goes on 

 the shape and size of the velum become modified by the growth 

 of the paraphysis and the telencephalic plexus in front of it, which 

 absorb a large part of the paraphysal arch and at the same time 

 the cephalic wall of the velum. This corresponds to Burckhardt's 

 description (13). When these structures are well developed the 

 velum appears in the sagittal plane as a rather insignificant angle, 

 and becomes still more obscured by the hemispheres which grow 

 dorsally on either side of it so that it is much reduced towards the 

 median line. In more advanced stages it becomes still less evi- 

 dent and forms merely a lip bounding the caudal edge of the open- 

 ing of the paraphysis. These steps can be traced in figs. 8, 9, 

 and 12. In those forms which have a well developed diencephalic 

 plexus the velum is still further modified. In early stages the 

 velum forms a sharp angle with a cephalic and caudal wall. The 

 former as I have mentioned is taken up by the paraphysis and telen- 

 cephalic plexuses, while the caudal wall forms the cephalic por- 

 tion of the post velar arch and is taken up more or less in the 

 formation of the diencephalic plexus, fig. 22. The term post 

 velar arch was given by Minot (71) to that part of the roof of 

 the diencephalion extending from the velum to the epiphysal 



