METABOLIC GRADIENTS 



71 



region, and passing inward to form the mesoderm. 

 Most of the region of the embryo behind the head in 



(^1 





Figs. 16, 17. — Transverse sections of chick embryo at different 

 levels, to show developmental gradients. 



Fig. 14 and the extreme posterior region of the embryo 

 in Fig. 15 are at about this stage of development. 



