INTRODUCTION 



JEnl 



Ekt 



FIG. 6, A AND B. DIAGRAMMATIC TRANSVERSE SECTIONS THROUGH A DEVELOPING 

 VERTEBRATE EMBRYO. 



JJ, alimentary canal ; Eat, hypoblast, showing in Fig. A the thickening (Ch) which 

 will form the notochord ; Ch l (Fig. B), the notochord now constricted off from 

 the hypoblast ; UW, mesoblastic somite ; UG, primary urinary duct (pro- 

 nephric duct); A, aorta; 8pP, splanchnic and Sop, somatic mesoblast ; 

 Co, Ca-I, ccelome ; H, remains of the upper part of the ccelome in the 

 interior of the mesoblastic somites ; Ekt, 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. 



