PARAPH YSIS AND PINEAL REGION IN REPTILIA 337 



hat they were due to prohferation of cells along the edges of the 

 plate. He found however ''in early stages a continuous primitive 

 segmentation of the nervous sj^stem serially homologous through- 

 out head and trunk — the neuromeric segmentation. In later 

 stages there appears in the encephalon a secondary (in time) seg- 

 mentation resulting in the so-called vesicles, which are not serially 

 homologous with the segments of the myelon, but give rise to an 

 anterior cephalic tract which is a region sui generis." He gives 

 two tables showing the number of segments determined by pre- 

 vious investigators and their relation to the vesicles of the brain 

 and to the nerves. He agrees with Orr as to the structure and 

 number of hind brain neuromeres, finding five. As regards the 

 neuromeres in the mid brain and fore brain there is a considerable 

 difference of opinion and Neal thinks that most authors have 

 counted dorsal expansions here which are really secondary sub- 

 divisions. He says that ''Morphologically different structures 

 have been described by them as neuromeres or encephalomeres and 

 that the divergence in their results does not seem to justify this 

 assumption." As hind brain neuromeres involve dorsal, lateral and 

 ventral zones, fore brain neuromes should do the same if they are 

 morphologically equivalent. If they do not then one should be 

 able to explain how that condition has been lost or modified. At 

 an early stage, (74) fig. 45, pi. 7, he finds six vesicles in a para- 

 sagittal section of the cephalic plate. I = fore brain in region 

 of the optic vesicles. 11= the mid brain. Ill = 'Hinterhirn' 

 and IV, V, and VI are hind brain neuromeres. 



Neal's fig. 47, a parasagittal section of a slightly older stage, 

 shows five expansions in the fore and mid brain. I = prosencephalon 

 and II = Kupffer's parencephalon and supports the epiphysis. 

 The mid brain shows three exjDansions. I = that part of brain 

 which later carries the posterior commissure. It seems to me 

 that Neal should classify this first mid brain segment as the sec- 

 ond diencephalic segment or synencephalon of Kupffer. This 

 picture then would correspond to my figs. 28, 30, 32, 34 and 36. Fig. 

 52, a frontal section of fig. 47, shows these divisions well and 

 corresponds to my figs. 29, 31 33, 35 and 37. According to my in- 

 terpretation we would have here then three fore brain and two mid 



