PARAPHYSIS AND PINEAL REGION IN REPTILIA 331 



gin of the lateral plexus on each side of the opening of theparaphy- 

 sis. This paired plexus is presumably homologous with the single 

 plexus of amphibians which, arising from the telencephalon 

 immediately anterior to the paraphysis, fills up the lower part 

 of the diencephalon and is known as the telencephalic choroid 

 plexus or plexus inferioris. Here it does not extend so far into 

 the diencephalon as is the case in Necturus for example, Warren 

 (97, fig. 16). 



That part of the cavity of the diencephalon the roof of which is 

 formed by the post velar arch is now filled up by a large mass of 

 choroid plexus, D. C. P. The plexus begins close to the opening 

 of the paraphysis but reaches its greatest development at the top 

 of the diencephalon. In the model one sees a large rounded 

 mass of plexus just below the roof. This is really the plexus of the 

 right side of the brain and a corresponding mass is hidden from 

 view on the opposite side. The diencephalic plexus therefore 

 tends to be paired or double as was the case with the telencephalic 

 plexus mentioned above, fig. 26, D.C.P. This is also in striking 

 contrast, as was the case with the telencephalic plexus just men- 

 tioned, to the amphibian type where its homologue is represented 

 by a single median mass, Warren (97, figs. 11, 13, 14). The 

 epiphysis has increased in length as well as in breadth especially 

 at the tip where it is quite expanded laterally. It is still a hollow 

 tube, though attached to the brain by a solid stalk. Like the 

 paraphysis it is closely moulded over the roof of the diencephalon. 

 There is not much to add about the commissures. They are 

 naturally larger especially the posterior which now shows a 

 slight angle on its dorsal surface. Its posterior limit, as here 

 marked, is purely arbitrary, since the fibers are spread out dif- 

 fusely over the outer layers of the mid brain wall. The outline 

 of this model is not quite correct in as much as the posterior half 

 of the model is carried too high above the anterior. Figs. 24, 25 

 and 26 are three sections of this model to illustrate the relations 

 of the parts which cannot be seen clearly in the model. They 

 correspond to the lines A-B, C-D, E-F, fig. 22. Figs. 25 and 

 26 show how the cephalic wall of the dorsal sack has been carried 

 forward on either side of the median line so as to form two recesses, 



