92 JOHN WARREN 



the diencephalon forms a large choroidal pouch overhanging 

 the telencephalon and suggests that this is the homolog of the 

 paraphysis of other vertebrates. In figure 22 a picture corre- 

 sponding to this description appears, but as the pouch is on the 

 diencephalic side of the velum it cannot be compared with the 

 true paraphysis which always is of telencephalic origin. His 

 (11), figure 56, shows a frontal section through that part of the 

 diencephalon prolonged forward over the velum, in which a few 

 choroidal folds appear, but no trace of any tubules or diverticuli 

 and the picture corresponds closely with that seen in the sections 

 of the embryo modeled in figure 22. 



As regards the appearance of this formation in lower verte- 

 bra;tes, there is but little evidence on which to base conclusions. 

 I have examined carefully the Harvard Collection of models 

 of the forebrain of Necturus, Lacerta (agilis and muralis), 

 and Chrysemys marginata. In all of these specimens the di- 

 encephalic roof plate is invaginated by the plexus formation in 

 the roof of the forebrain. Here one finds solid projections into 

 the brain cavity instead of hollow tubules or diverticuli growing 

 out of that cavity. This is especially the case in Amphibia, 

 where practically all of the postvelar arch is absorbed into a 

 huge mass of plexus, which extends back to the hindbrain. In 

 Chrysemys there is a slight projection of the diencephalic roof 

 plate forward on either side of the paraphysis, but this seems to 

 be due rather to the impression made by the latter on the dien- 

 cephalic roof and practically the whole roof plate is invaded 

 by masses of plexus. In Lacerta the plexus formation affects 

 the roof of the diencephalon in the neighborhood of the epiphysis 

 only and the velar portion is absolutely smooth and flat. In birds 

 Dexter (4), figures 5 and 7, shows an oval or a triangular shaped 

 vesicle lying dorsal to the paraphysis and close against the wall 

 of the diencephalon. He finds this as an inconstant structure 

 in chick embryos from 60 mm. up to young birds after birth. 

 He mentions also the presence of detached vesicles referred to 

 by Dendy in Sphenodon and by other authors in some of the 

 Lacertilia. These latter, however, all seem to belong to the 

 epiphysal part of the forebrain roof. Whether the vesicle seen 



