208 



SENESCENCE AND REJUVENESCENCE 









1! ^'^ 









1 



Figs. 73, 74. — Axial developmental gradients in the chick embryo: Fig. 73, 

 showing the formation of the mesoderm, is from the posterior region of the same 

 embry'o as Fig. 74, from a more anterior region, in which morphogenesis has extended 

 both laterally and ventrally from the mid-dorsal region. From embryological prep- 

 arations of the University of Chicago. 



