318 JOHN WARREN 



as an angle in the roof, and behind it the post velar arch forms a 

 very striking dome in the diencephalic roof. The separation 

 between the two pineal outgrowths is more marked than in the 

 preceding stages, and a median section would show that the 

 future pineal eye is now nearly separated from the epiphysis. 

 Caudad to it is the pars intercalaris (synencephalic arch), S, 

 which has now been invaded in its caudal part by the beginning 

 of the posterior commissure, P. C. It can be seen that this com- 

 missure lies almost entirely in this part of the diencephalic roof 

 and does noi extend backward beyond the dividing line between 

 diencephalon and mesencephalon. It would therefore appear as 

 if this commissure belongs — at this early stage at least — to the 

 diencephalon, and that it invades the mesencephalon only at a 

 later period of development. At this stage we have several new 

 structures developing from the primary arches in the roof of the 

 three subdivisions of the fore brain: the paraphysis arising from 

 the paraphysal arch in the roof of the telencephalic ; the epiphy- 

 sis and pineal eye from the epiphysal arch in the roof of the first 

 diencephalic; and the posterior commissure in a part of the roof 

 of the second diencephalic segment. Further changes will con- 

 sist in the appearance of the telencephalic or lateral choroid 

 plexus from the telencephalic segment and the diencephalic 

 plexus and superior commissure from the first diencephalic segment. 

 The posterior commissure will eventually occupy all the roof of 

 the second diencephalic segment. 



Fig. 9 shows the brain of Lacerta muralis of 5 mm. A very 

 marked increase in the development of all parts is apparent. 

 The paraphysis is now a large vesicle with a wide mouth and at 

 its apex are two distinct outgrowths while near its base, on the pos- 

 terior side, is a third. Its general inclination is upward and back- 

 ward, tending to follow the curve of the post velar arch. The 

 velum is not as sharply marked as in the earlier stages and forms 

 rather a wide angle. The post velar arch occupies the greater 

 part of the roof of the diencephalon, forming a high, wide dome, 

 the brain roof here being extremely thin and formed only by a 

 single layer of cells. The pineal eye, P. E., is now a rounded ves- 

 icle, completely separated from the epiphysis, Ep., but is, how- 



