INTRODUCTION 



VTT 



Mnd 



m — ^aP 



Ji'n/ 





Fni- 



Fig. 6, A and B. 



-Utct 



C'oel 



-Diagrammatic Transverse Sections throucih a developing 

 Vertebrate Embryo. 



D, alimentary canal ; Eid, liypoblast, showing in Fig. A the thickening (Ch) which 

 will form tlie notochord ; GIO- (Fig. B), tlie notochord now constricted off from 

 tlie hypoblast; V)Y, mesoblastic somite; XJO, primary urinary duct (pro- 

 nephric duct) ; A , aorta ; iSpP, splanchnic and Sop, somatic mesoblast ; 

 Co, (Joil, coelome ; H, remains of the upper part of the coelome in the 

 interior of the mesoblastic somites ; EM, epiblast ; Med, central nervous 

 system (medullary cord) : — in Fig. A it is shown still connected with the 

 epiblast, from which it has become constricted off in Fig. B. 



