EMBRYOLOGY 



407 



op. y&s. 



/tA. Afes 



V.o.m. 



n.T. 



give rise to the lamina 

 terminalis. The fusion of 

 the neural folds proceeds 

 more rapidly in the pos- 

 terior direction, although 

 the extreme hinder ends 

 remain apart for a con- 

 siderable time, and as it 

 does so a second constric- 

 tion forms. The first con- 

 striction marks off the 

 fore-brain vesicle or pro- 

 sencephalon, and the 

 second, the mid - brain 

 vesicle or mesencephalon, 

 while the remaining part 

 of the brain region is 

 termed the hind - brain 

 vesicle or rhombencepha- 

 lon. Even at this early 

 stage the ventro - lateral 

 walls of the fore-brain have 

 bulged out laterally to 

 form the rudiments of the 

 optic vesicles. Closer ex- 

 amination shows that 

 underlying this primary 

 division of the brain there 

 is an indication of segmen- 

 tation, so that the brain 

 itself may be regarded as 

 consisting of a .series of 

 segments or neuromeres, 

 marked off from one 

 another by faint constric- 

 tions. Three such neuro- 

 meres are generally held 

 to be present in the fore- 

 brain, two in the mid- 

 brain, and in the hind- 

 brain five can fairly easily 

 be distinguished, of which 

 the hindermost is the largest and usually considered to represent two. 

 The total number of neuromeres in the chick therefore is eleven. 



Met,. 



pr.str. 



FIG. 142. 'Chick, seven pairs somites. 

 "*From Lillie. 



a.c.s., anterior cerebral suture ; ceph.Mes., cephalic 

 mesoblast ; F.G., fore-gut ; N'ch., notochord ; n.T., 

 neural tube; op.Ves., optic vesicle; pr'a., proamnion ; 

 pr.str., primitive streak ; sz., 57., second and seventh 

 somites ; V.o.m., omphalo-roesenteric vein. 



