THE LATER DEVELOPMENT OF THE CHICK 305 



immediately in front of it, not now distinct but differentiating 

 later, together mark, morphologically, the true anterior end 

 of the central nervous system. 



The anterior and lateral portions of the telencephalon now 

 begin to expand, giving the effect of a constriction at its 

 posterior limit. This apparent constriction of the dorsal wall 

 of the fore-brain is the velum transversum, which marks the 

 separation between telencephalon and diencephalon. In these 



lo. 



FIG. 124. Median sagittal section through the brain of the chick of twelve 

 to thirteen days. From Von Kupffer (Hertwig's Handbuch, etc.) ; c, Cerebellum; 

 ca, anterior commissure; cd, notochord; ch, habenular commissure; ci, infundi- 

 bular commissure; ck, central canal of spinal cord; cp, posterior commissure; 

 cpa, anterior pallial commissure; cs, spinal commissure; cv, cavum cerebelli; 

 cw, optic chiasma; dr, epiphysial gland; dt, decussation of the trochlear (IV) 

 nerve; e, epiphysis; ei, paraphysis; hm, cerebral hemisphere; hy, hypophysis; 

 J, infundibulum; le, ependymal lamina of the roof of the fourth ventricle; lo, 

 olfactory lobe; Ip, posterior lobe of cerebral hemisphere; M, mesencephalon; 

 opt, optic chiasma; pch, choroid plexus third ventricle; pi, choroid plexus of fourth 

 ventricle; re, epiphysial recess; ro, optic recess; s, saccus of infundibulum; si, 

 posterior intracephalic furrow; tp, tuberculum posterius; tpi, tuberculum mam- 

 millare; tr, torus transversus; va, velum medullare anterius; vi, median ventricle 

 of telencephalon; vp, velum medullare posterius. 



early stages no other sharp distinction between these two 

 regions can be determined. 



Toward the close of the second day a pair of lateral exten- 

 sions of the telencephalon push out rapidly and expand dor- 

 sally, anteriorly, and posteriorly; these are the rudiments of 



