PARAPHYSIS AND PINEAL REGION IN REPTILIA 339 



that these had the same morphological value as the early segments 

 owing to their late appearance and their dorsal lines of division. 



Hill's (42) fig. 3 shows the line between the first and second 

 neuromeres splitting to surround the optic vesicle. In fig. 10 

 the optic vesicle is clearly assigned to the second neuromere. 

 Figs. 40 and 41 show the three secondary subdivisions of the fore 

 brain similar to my model, fig. 3. In fig. 40 the optic vesicle 

 seems to belong to the first segment or the telencephalon. Fig. 

 42 is a parasagittal section of Salmo purpuratus showing five seg- 

 ments to fore brain and very similar to my figs. 28, 30, 32, 34 and 

 36. The third segment is wedge shaped, broad above, but narrow 

 below at the habenular angle. 



Weber (99) studied the segmentation of the brain in the pheas- 

 ant shortly before and after the closure of the neural tube and 

 made models of various stages. In his younger stages there were 

 four neuromeres in the fore brain. The first is the lobus olfac- 

 torius impar, the second is the telencephalon. On its roof are 

 the hemispheres, on its lateral walls the optic vesicles and in the 

 floor the saccus vasculosus. The third is Kupffer's parencephalon 

 with which the eye communicates and is broad dorsally and nar- 

 row below at the tuberculum posterius as in Hill's fig. 42. This 

 one has the epiphysis on its roof and the recessus mammillaris 

 in the floor. The fourth he calls the ''diencephalon" bounded 

 below by the interpeduncular eminence. 



Weber finds that these segments are clearly marked off by dor- 

 sal and ventral folds and he assigns to them a metameric value. 

 His figs. 4, 5, 10, and 11 show these four neuromeres in the pheas- 

 ant. In my models Weber's I and II form the first neuromere 

 with the optic vesicle arising from it. I have been unable to 

 detect any sign of subdivision in this first segment such as he 

 shows. In the chick he finds five neuromeres in the fore brain 

 (see text figs. 5 and 6). 



Mrs. Gage (33) in a study of a three weeks old human 

 embryo describes certain total folds in the fore brain radiating down- 

 ward from the membranous roof along the lateral wall. These do 

 not seem to correspond to any of the folds seen in the turtle or 

 lizard. 



