354 JOHN WARREN 



that in Necturus, fig. 20. In lizards the post velar arch is at first 

 low but soon develops up and back to form a large dome like cav- 

 ity in the upper part of the diencephalon, figs. 3-10. This is even 

 more marked in the turtle, figs. 17-21, Coincident with this 

 process in both cases is the growth of the paraphysis, and the 

 velum becomes reduced here to a mere lip or border bounding the 

 mouth of the paraphysis. In the lizards after the stage shown in 

 fig. 10 the cephalic and caudal walls of the extensive post velar 

 become steadily forced towards each other by the mid brain en- 

 croaching from behind and the hemispheres from in front. The 

 end result is seen in fig. 13. The velum at this stage becomes more 

 prominent and rounded by the ingrowth of the fibers of Elliott 

 Smith's aberrant commissure, figs. 12 and 13; Kupffer (57), fig. 

 248, Anguis fragilis; fig. 265, Lacerta vivipara. In the lizard the 

 plexus formation is confined to the apex of the diencephalon, 

 which has now been reduced to a slit very narrow antero-poste- 

 riorly, the superior commissure being separated by a slight interval 

 only from the velum. The velum, however, is quite distinct and 

 also a large part of the post velar arch, which has not entered into 

 the formation of the diencephalic plexus. This closing up of the 

 post velar arch or dorsal sack is one of the most striking features 

 in the development of the fore brain of the lizard. If we compare 

 this condition with my oldest turtle, which represents probably 

 quite closely the final stages, we find a very different picture. 

 The velum does not contain a commissure as was the case in the 

 lizard and remains an insignificant, narrow lip, which becomes 

 still further obscured by the development of the diencephalic 

 plexus, figs. 20-23. There is no reduction in the size of the dorsal 

 sack, which remains an extensive vaulted structure, but it is 

 almost entirely filled up with an elaborate plexus extending from 

 the mouth of the paraphysis to the superior commissure, figs. 22, 23, 

 25, 26. Voeltzkow (96), fig. 14, shows an embryo of Caiman 

 niger that resembles the lizard of 37 mm. fig. 12. The velum 

 a mere lip, with no commissure, the dorsal sack deep and fairly 

 narrow with a diencephalic plexus at its apex only. The superior 

 commissure lies very near the velum as in the lizard. 



