PARAPHYSIS AND PINEAL REGION IN REPTILIA 359 



at the same stage in which the superior commissure first appears. 

 It is a curious fact that this is also the case in Necturus and in 

 the turtle, fig. 10. It forms here at first a rather solid mass, which 

 soon, however, takes on the characteristic form of the choroid 

 plexus. In the adult lizard it has rather a peculiar shape. It 

 begins as a long narrow rounded mass just anterior and lateral 

 to the paraphysis, which broadens out in the lateral ventricle. 

 Its long axis is in the dorso-ventral direction, it being relatively 

 narrow in the antero-posterior plane. Figs. 10-13 show the steps 

 in its development. In the turtle this plexus is more developed 

 and appears at earlier stages. It forms a large irregular mass 

 roughly oval or quadrilateral and much more extensive than in 

 the lizard. Its development is shown in figs. 20, 21, 22, and in 

 section in figs. 25 and 26. In birds and mammals, especially in 

 the latter, it is highly developed. See Ziehen, figs. 13, hedgehog 

 embryo, 32 Echidna, 52-56 rabbit, 82 cat; Minot (71), fig. 390, 

 sheep. 



The diencephalic plexus develops from the caudal wall of the 

 velum and from the post velar arch. This plexus is not well 

 developed in lower forms and is absent in Petromyzon. It is 

 probably represented by the thin membranous roof of the dien- 

 cephalon in this and other lower vertebrates, Studnicka (93). 

 In Acanthias, Accipenser and Amia it is seen in those folds that 

 appear on the caudal wall of the velum (see p. 357). Burck- 

 hardt (13) describes it in Protopterus. In amphibia its develop- 

 ment is excessive. It takes in all the caudal wall of the velum 

 and the greater part of the post velar arch and extends backwards 

 as a broad median mass, wide from above downward, but narrow 

 from side to side, to end in a tufted extremity in the fourth ven- 

 tricle. Warren (97), figs. 13, 16, and 23, shows this in Necturus, 

 Burkhardt (13) in Ichthyophis, Mrs. Gage (32) in Diemyctylus, 

 figs. 6, 23-25. In the lizard this plexus is represented by a series 

 of transverse folds in the roof of the dorsal sack, fig. 12, and the 

 plexus here is relatively small except at the adult stage, fig. 13. 

 See also Francotte (29, 30), Kupffer (57), fig. 248, Anguis fragilis, 

 265 Lacerta vivipara, 258 Ornithorhyncus (G. E. Smith), Voeltz- 

 kow (96), Crocodilia, figs. 10, 11, 14, 15. 



