FIELDS AND GRADIENTS IN NORMAL ONTOGENY 343 



the temperature-gradient was concluded prior to the onset of 

 gastrulation, we do not know whether the primary gradient-system 

 may not also be directly deformed (fig. 161). See Appendix, p . 494. 



^J 



Fig. 162 



Effects of a lateral temperature-gradient (3 days from beginning incubation) on 

 the development of the chick. Above, the whole blastoderm on the heated (right) 

 side, the area vasculosa much larger and more differentiated, the optic vesicle 

 moderately larger. The somites on the heated side have been "stepped up" so 

 as to alternate with those on the other side: this is shown below on a larger scale. 

 (From Tazelaar, Quart. Journ. Micr. Sci.-hxxu, 1928.) 



Somewhat similar results were obtained by applying tempera- 

 ture-gradients to chick embryos. In some specimens treated with 

 lateral gradients, the mesoblastic somites on the heated side were 



