74 



DYNAMICS OF HISTOGENESIS. II 



6 7 8 9 10 11 fZ 15 1¥ f5 U 17 18 19 Z/> Zf ZX Z5 Z^ 



Fig. 17. Curves of differential growtli of the tracheal epithelial tube and 

 mesenchyme. The greater growth of the epithelial tube over that of the 

 mesenchyme after the embryo has reached a length of 11 mm. is shown graphi- 

 cally. This rapid growth causes a progressive compression of the mesenchyme 

 acting, therefore, as the efficient stimulus in the differentiation of cartilage tissue. 

 The tracheal embryonic cartilage appears in embryos between 18 and 24 mm. in 

 length. Note particularly that during this period the curve of epithelial tubular 

 growth in area rapidly ascends. 



