FIELDS AND GRADIENTS IN NORMAL ONTOGENY 339 



§ 6 . The effects of temperature- gradients 



In Amphibia, too, the primary gradient can be experimentally 

 modified in various ways. One is by superimposing a temperature- 

 gradient upon it during early development. This has been effected 

 by several different methods. ^ The gradient may be applied in 

 various directions, e.g. from side to side across the main axis 

 (lateral) or along it (polar). In the latter case, the temperature- 

 gradient may be adjuvant to the egg's original gradient, or else 



Fig. 158 

 Effect of a lateral temperature-gradient, applied for s\ hours from the 2-cell stage, 

 on cleavage in the frog. The animal cells are larger on the right (cooled) side, small 

 on the left (heated) side. Note the sharp demarcation between large and small 

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



antagonistic. In the former case, the difference in size between 

 blastomeres of the animal and vegetative hemispheres is accentuated 

 at the close of cleavage, whereas in the latter it is reduced, often 

 to the extent of leaving the animal blastomeres scarcely smaller than 

 those at the vegetative pole. Various minor anomalies of gastru- 

 lation are produced, but the net result of adjuvant temperature- 

 gradients is the production of embryos and young larvae with 

 somewhat oversized heads, whereas, with antagonistic gradients, 

 the head region is subnormal. This shows the plasticity of the 

 ^ Huxley, 1927; Castelnuovo, 1932. 



