VI.] 



THE CRANIAL AND SPINAL NERVES. 



125 



round the alimentary canal to reach the mesoblast above it, 

 AOA and AO woxild have formed one continuous curved 

 space. In sections lower down in the back the two aortse, 

 AO, one on each side, would be found fused into one median 

 canal. 



an examination of Fig. 41, where the two roots of the 

 vagus nerve (vg) are shewn growing out from the neural 

 band. Shortly after this stage the neural band becomes 

 separated from the external epiblast, and constitutes 



FlQ. 41. 



Transvebsb Section through the Posterior Part of the 

 Head of an Embryo Chick of Thirty Hours. 



hb. hind-brain ; vff. vagus nerve ; ep. epiblast ; ch. notochord ; 

 X. thickening of hypoblast (possibly a rudiment of the sub- 

 notoohordal rod) ; al. throat ; ht. heart ; pp. body cavity ; 

 so. somatic mesoblast ; sf. splanchnic mesoblast ; Ay. hypo- 

 blast. 



