340 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



Fig. 159 



Effects of temperature-gradients applied to frogs' eggs from soon after fertilisa- 

 tion, a, b, Preserved during gastrulation. a, After adjuvant gradient for 12 hours : 

 note marked overgrowth by the dorsal lip and absence of the ventral lip. b, After 

 antagonistic gradient for 16 hours. Note larger animal and smaller yolk-cells, 

 presence of ventral lip, and slight overgrowth by dorsal lip. c and d, Preserved as 

 early neurulae. c (below), After adjuvant gradient for 12 hours. Note small 

 neural folds (see text), no ventral differentiation of mesoderm, d (above). After 

 antagonistic gradient for 16 hours. Note large neural folds (see text), well- 

 formed ventral mesoderm but poor differentiation of notochord and myotomal 

 mesoderm. (From Dean, Shaw and Tazelaar, Brit. Journ. Exp. Biol, v, 1928.) 



