346 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



are produced by a wide variety of depressant substances. This fact 

 is characteristic of the experimental modification of gradients : any 

 specific effect of the agent employed is usually overridden by its 

 general effects which are exerted on the shape and the slope of the 

 gradient (but see p. 337). Similar results have been obtained with 

 Anura.i i^ toads, remarkable malformations of the mouth region 

 are to be noted (figs. 165, 168). 



Fig. 164 

 DiflFerential susceptibility in the early development of the frog. A, Differential 

 acclimatisation of frog embryo exposed for 4 days from fertilisation to very dilute 

 KCN. Note very large head. B and D, Differential acceleration. Frog embryos 

 after 4 and 6 days respectively in NI5000 HCl from the 2-cell stage. Note rela- 

 tively large head and accelerated development, as against control at 6 days (C). 

 (Redrawn after Bellamy, Amer.Joiini. Anat. xxx, 1922.) 



In moderate concentrations, the result of exposure of Fundulus 

 eggs to depressant substances is the production of a head of reduced 

 size, the reduction being disproportionately great in the inter- 

 ocular region— in other words, the exact converse ^f the experi- 

 ments with stimulants. But when more marked effects are pro- 

 duced, they consist in the complete non-formation of a greater or 



1 Bellamy, 19 19; Cotronei, 1921. 



