INTRODUCTION 



JZnt 



SoP 



Cod 



FIG. 6, A AND B. DIAGRAMMATIC TRANS VERSE SECTIONS THROUGH A DEVELOPING 

 VERTEBRATE EMBRYO. 



^4, aorta ; Ch 1 , (Fig. B), the notochord now constructed off from the endoderm ; 

 Co, Civlj ccelome ; /), alimentary canal ; Ekt, ectoderm ; Ent, endoderm, 

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

 remains of the upper part of the co?lome in the interior of the mesodermic 

 somites ; J\ferl, central nervous system (medullary cord) : in Fig. A it is 

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

 off in Fig. B ; SoP, somatic, and tfpP, splanchnic mesoderm ; UG, primary 

 urinary duct (pronephric duct) ; UW. mesodermic somite. 



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