PARAPH YSIS AND PINEAL REGION IN REPTILIA 341 



third fore brain segment appears here relatively much later than 

 in the lizard and turtle where it was one of the first structures to 

 appear in the roof of the fore brain. 



In my earliest models the fore and mid brain vesicles are clearly 

 shown but there is no trace of any neuromeres in this region. In 

 the hind brain of these specimens not shown in the model, the 

 neuromeres were well marked. The first sign of subdivision of 

 the fore brain of the lizard is shown in fig. 2, where there is just 

 the beginning of the velum separating telencephalon from dience- 

 phalon and in the roof of the latter division is seen the short arch, 

 S, lying just anterior to the mid brain and forming the roof of the 

 synencephalic segment. In the turtle fig. 16 the velum has not 

 appeared and the arch forming the roof of the synencephalic seg- 

 ment alone is seen. The features are shown more clearly in the 

 next models, figs. 2, 3, 4, 17 and 18. Here the internal ridge con- 

 tinuous with the velum marks the caudal limit of the telencephalon 

 and ends in the floor just behind the optic stalk. In fact it forms 

 a distinct dorsal boundary to the optic stalk. This seems to me 

 to assign the optic vesicle clearly to the telencephalon and not to 

 the diencephalon as stated by others (see Hill (42), Kupffer (57), 

 and Johnson (53)). 



In more advanced stages, figs. 5, 7, 8, 18 and 19, the hemi- 

 spheres are seen gradually bulging outward above the optic stalk. 

 There appears now another ridge above and in front of the optic 

 stalk, and apparently continuous above the velum. This might 

 seem to mean that the optic vesicles did not belong to the telen- 

 cephalon but lay behind its caudal limit in the front part of the 

 diencephalon. This new ridge is, however, secondary and is due 

 to the deepening of the hemisphere and also of the optic stalk. 

 The ridge first mentioned and shown in figs. 2, 3, and 17 is really 

 the primary line of separation between telencephalon and dien- 

 cephalon and the optic stalk lies clearly in front of it. This 

 first subdivision of the primary fore brain is the telencephalon 

 and contains the eye vesicles. In its roof appears the paraphysal 

 arch and later the paraphysis and the telencephalic plexuses. Its 

 caudal limit is the velum and the internal ridge continuing the 

 velum, which ends in the floor just behind the optic recess at the 



