248 



THE DEVELOPMENT OF THE CHICK 



from the recessus opticus to the region between the foramina of 

 Monro. It is very thin, excepting near its center, where it is 

 thickened to form the torus transversus, containing the anterior 

 commissure. At its dorsal summit it is continuous with the 

 roof of the telencephalon medium, which has formed a pouch- 

 like evagination, the paraphysis. Just behind the paraphysis 



Fig. 149. — Median sagittal section of the brain of a chick embryo of 7 



days. (After v. Kupffer.) 



c., Cerebellum, ca., Anterior commissure, cd., Notochord. ch.. Pro- 

 jection of the optic chiasma. cp., Posterior commissure, e., Epiphysis, 

 e'., Paraphysis. hy., Hypophysis. I., Infundibulum. It., Lamina termi- 

 nalis. Lop., Optic lobe. M., Mesencephalon. Mt., Metencephalon. 

 opt., Chiasma of the optic nerves, p., Parencephalon. ro., Recessus 

 opticus, s., Saccus infundibuli. se., Synencephalon. tp., Mammillary 

 tubercle, tp., Tuberculum posterius. tr., Torus transversus. Tr., De- 

 cussation of the trochlear nerves. Va., Velum medullare anterius. Vi., 

 Ventriculus impar telencephali. vp., Velum medullare posterius. 



is the velum transversum, where the roof bends upwards sharply 

 into the roof of the diencephalon. The epithelial wall around 

 the bend is folded to form the choroid plexus of the third ven- 

 tricle, which is continued forward into the lateral ventricle along 



