488 'Journal of Comparative Neurology and Psychology. 



practically the same as that of the transverse velum. The velum 

 does not become prominent until after the optic vesicle is well 

 formed (stage K), but in some specimens a slight fold representing 

 it can be recognized in models of embryos as early as 24 somites or 

 earlier. I am inclined to think that the velum represents an infold- 

 ing of the brain wall which is begun early on account of the with- 

 drawal of material from the alar plate to form the optic vesicle. 

 It is the second neuromere whose dorsal half thus gives rise to the 

 retina, while its ventral half becomes depressed and bulged ventro- 

 caudally to form the primitive inferior lobes above referred to. 



3. Ganoids and Teleosts. 



The diencephalon presents no features of especial importance. 

 There is a greater development of the inferior lobes than in sela- 

 chians, although the olfactory apparatus is of less importance. This 

 is doubtless to be attributed to the much greater importance of the 

 gustatory apparatus in ganoids and teleosts. The saccus vasculosus 

 reaches a very great development in some of these forms and it has 

 been shown that there is an intermingling of the epithelial sacs of 

 the saccus with those of the hypophysis. The optic tracts form a 

 chiasma in the floor of the brain in some forms and in others cross 

 at some distance from the brain. 



The telencephalon presents certain great peculiarities. It is usu- 

 ally somewhat more elongated than that of most selachians and re- 

 sembles that of Chimsera or Heptanchus. Also the telencephalon 

 has no massive roof, but only a broad membraneous tela continuous 

 with that of the diencephalon. The boundary between the di- and 

 telencephalon in the roof is marked by a velum transversum which 

 forms the front wall of the dorsal sac of the diencephalon. 



The membraneous roof of the telencephalon is much more exten- 

 sive than in selachians or other vertebrates. In many cases the lateral 

 walls of the telencephalon are rolled outward (laterad) so that the 

 morphological dorsal border is directed laterad or latero-ventrad. 

 This makes the membraneous roof in these fonns exceedingly broad. 

 The ventricle is correspondingly extensive and toward its anterior 

 end divides into lateral ventricles which extend into the olfactory 



