496 'Journal of Comparative Neurology and Psychology. 



SL membraneous tela over the median ventricle and this is continued 

 as the roof of the interventricular foramen into the wall of each 

 hemisphere. In front of the foramina the tela meets the lamina 

 terminalis, so that as in amphibians there is a membraneous tela in 

 the median region of the telencephalon for its whole length. For the 

 morphological relations of this tela, the ventricles and the chorioid 

 plexuses it is necessary to study the embryolog}^ 



While in all lower classes, except cyclostomes, a prominent velum 

 transversum marks the boundary between diencephalon and telen- 

 cephalon, in mammals the velum has not heretofore been recognized. 

 The large collection of pig embryos in this laboratory gives excel- 



FiG. 34. Pig embryo of 5 mm. Model of right half of head seeu from 

 medial sm-face. The optic vesicles are still connected with the primitive 

 optic groove. The Roman numerals indicate the brain neuromeres. x 2.5. 



lent opportunity for comparison with the lower classes described 

 above. 



The earliest stage available is a 5 mm. pig cut in transverse series 

 from which a model of the brain has been made (Fig. 34). From 

 the figure it will be seen that this brain agrees very closely with 

 that of the Squalus embryo of about 20 somites. The cavity of the 

 optic stalk is continuous with a groove which traverses the median 

 line, the primitive optic groove. Behind this is the primitive inferior 

 lobe, a ventral expansion bounded caudally by the tuberculum pos- 

 terius. In front of the primitive optic groove is a transverse ridge 

 whose cross section in the median plane presents the form of an arch. 



