PARAPHYSIS AND PINEAL REGION IN REPTILIA 315 



velum and ending behind the optic stalk, 0. S. That part of the 

 brain cephalad to this ridge is the first segment of the fore brain or 

 the telencephalic neuromere of Kupffer. The roof of this segment 

 forms a slight arch in front of the velum. This is the beginning of 

 the paraphysal arch, P. A., from which the paraphysis will develop 

 at a later stage. The optic vesicle grows out from the ventro-lat- 

 eral wall of this segment. Behind the velum the roof of the dien- 

 cephalon forms a sHght curve ending at a small but very distinct 

 arch, *S. This division of the roof, belongs to the second segment of 

 the fore brain or the first diencephahc segment, /. D. The segment 

 is limited cephalad by the ridge passing from the velum to the 

 optic stalk, and caudad by a slight ridge passing from the cephalic 

 end of the small arch mentioned above to the habenular angle, 

 or flexure, H. F., at the base of the brain. This segment is Kupf- 

 fer's (57) parencephalic neuromere, or parencephalon, and from 

 its roof will develop later the post velar arch, epiphysal arch, and 

 the superior commissure. Between this segment of the dienceph- 

 alon and the anterior end of the mid brain we have a short seg- 

 ment, the roof of which forms the short, well-defined arch S. 

 This subdivision lies between the ridge forming the caudal limit 

 of the first diencephahc segment and the dorsal groove and lat- 

 eral ridge marking the cephahc Hmit of the mid brain, and is the 

 second diencephalic segment, Kupffer's (57) ' Synencephales 

 Neuromer des Diencephalon,' or synencephalon, II.D. Its 

 roof will form that part of the diencephalon lying between the 

 epiphysis and the mid brain. The posterior commissure wih 

 develop in the caudal end of this region, and eventually occupy 

 all of it. The ridges which separate the three segments of the 

 fore brain appear as grooves on the outer surface. At the pres- 

 ent stage, therefore, we have the three sub-divisions fairly well 

 defined, but of the structures that develop from the roof of each 

 division we recognize only the anlage of the velum. It should be 

 noted, however, that the roof of the second diencephalic segment 

 at this stage is very well defined, and is therefore one of the earli- 

 est portions of the roof of the fore brain to be <?learly differen- 

 tiated. 



