152 



THE DEVELOPMENT OF THE CHICK 



The median strip includes the tela choroidea, beginning at 

 the diencephalon, and the lamina terminalis, which ends at the 

 recessus opticus. These divisions are of great prospective signifi- 

 cance, though at the stage of 36 s they are but slightly differen- 

 tiated, save by their position. A slight thickening of the lamina 

 terminalis just in front of the recessus opticus marks the site of 

 the future anterior commissure (Figs. 87 and 88). 



Mesenc 



Me/enc. 



/ 



Mi^eJenc 



\-r C/) Opt. 



f~-/T'PC Opt 



I 



D/enc - 



V.tr \^ 

 Te/enc 



Fig. 86. — Inner view of the brain of a chick of about 82 hours, drawn from 

 a dissection. 

 Ch. opt., Chiasma opticus. Ep., Epiphysis (pineal gland). Isth., Isth- 

 mus. PI. enc. v., Plica encephali ventralis. Rec. opt., Recessus opticus. 

 V. tr., Velum Transversum. Other abbreviations as before. 



The Diencephaloii. The portion of the primary fore-brain pos- 

 terior to the telencephalon is known as the diencephalon. It in- 

 cludes the second and third neuromeres and probably also the 

 ventral zones and floor of the first (Fig. 83). A slight constriction 

 in the roof that appears about the 18 to 20 s stage near the junc- 

 tion of the middle and last third may represent the boundary be- 

 tween the second and third neuromeres; this persists for a long 

 time and may be traced in the lateral walls to the region of the 



