DIFFERENTIATION OF NERVOUS STRUCTURE 1 1 1 



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FIGS. 16, 17. Transverse sections of chick embryo at different 

 levels, to indicate developmental gradients: Fig. 16 indicates the condi- 

 tion of the posterior half or more of the stage of Fig. 14 and of the 

 extreme posterior region of the stage of Fig. 15; Fig. 17 indicates the 

 condition at the level of the sixth somite of Fig. 15; the region of most 

 intense developmental activity is primarily median dorsal, and develop- 

 ment progresses in general laterally and ventrally (from Child, 1915^). 



