INTRODUCTION 



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Fi<;. 6, A AND B. DIAGRAMMATIC TRANSVERSE SKCTIONS THROUGH A DEVELOMM; 



VERTEBRATE EMBRYO. 



A, aorta; Ch l , (Fig. B), the notochord now constructed oft' from the endoderm ; 

 Go, Co'l, ca'lome ; J), alimentary canal ; Ekf, ectoderm ; Ent, endoderm, 

 showing in Fig. A the thickening (Gh) which will form the notochord ; H, 

 remains of the upper part of the cu'lomu in the interior of the mesodei inic 

 somites; M<><1, central nervous system (medullary <-<>rd) : in Fig. A it is 

 shown still connected with the ectoderm, from which il has become constricted 

 off in Fig. B; So P, somatic, and $pP, splanchnic mesoderm ; U<!, primary 

 urinary duct (pronephric duct); U\V, mesodermic somite. 



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