DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 245 



Moreover, Rulon finds that, even with concentrations of thiocyanate 

 and exposure periods preceding fertihzation which produce the highest 

 percentages (30-40) of ectodermized or animahzed forms, the forms in 

 a given lot range from these through normal individuals to differentially 

 inhibited modifications. Also, with increase in concentration or exposure 

 period preceding fertilization, frequency of animalized forms decreases and 

 that of differential inhibitions increases. If animalization were a direct 

 effect of thiocyanate, increase in frequency of animalized forms up to 

 100 per cent, or nearly, would be expected with increase in concentration 

 and exposure, at least up to a certain limit. That is what happens as re- 

 gards frequency of exogastrulation produced by lithium. But after thio- 

 cyanate, even in the most favorable cases, less than half the individuals 

 exposed are animalized; and with increase in thiocyanate action beyond 

 this point animalization is progressively decreased, and modification in 

 the opposite direction, differential inhibition, increases. This is an effect 

 exactly similar to the relations between differential recovery and differ- 

 ential inhibition with various other agents and stages after fertilization 

 as concentration or exposure increases. Another point suggesting that the 

 direct effect of thiocyanate is differential inhibition before fertilization as 

 well as after is that in the more extreme animalizations ventrodorsality is 

 apparently completely obliterated, though more or less polarity may still 

 be present. Obliteration of ventrodorsality with polarity still an effective 

 factor in development is a characteristic result of certain degrees of differ- 

 ential inhibition with various agents, as many of the forms in Figures 

 91-96 indicate. In the light of all the data available Lindahl's conclusion 

 that animalization is a direct effect of thiocyanate treatment before fertili- 

 zation seems not entirely satisfactory. 



Experiments with sulphate-free sea water (Lindahl and Stordal, 1937), 

 together with other earlier experiments, have led to the hypothesis that 

 carbohydrate metabolism is characteristic of the animal region, protein 

 metabolism of the vegetal. Only one line of evidence regarded as support- 

 ing this hypothesis is mentioned here. Development is found to be more 

 modified in absence of sulphate than when it is present in the artificial sea 

 water used; from this it is argued that in sulphate-free water the develop- 

 ing embryos give off more substance that inhibits development than when 

 sulphate is present. Also, developing vegetal halves are injured by lack 

 of sulphate, animal halves are not: on this basis the hypothesis is ad- 

 vanced that the vegetal halves produce poisons which are rendered non- 

 toxic by sulphate. The possibility that absence of sulphate may itself 

 inhibit development seems not to have been considered. Moreover, if the 



