146 The Unity of the Organism 



ever, supplemented Gudernatsch's work by depriving the 

 larvae of thyroid altogether instead of giving them an extra 

 allowance through feeding. Allen extirpates the entire em- 

 bryonic thyroid from tadpoles long before any indications of 

 metamorphosis appear. What he finds of special impor- 

 tance for the present discussion is contained in the follow- 

 ing: 



"While Gudernatsch showed that thyroid feeding acceler- 

 ates development, this work shows that the total absence of 

 the th} 7 roid gland produces complete cessation of somatic 

 differentiation at a certain stage but does not hinder con- 

 tinued growth in size." 7 



O 



The first part of this statement taken alone might be 

 looked upon as confirmatory of the "formative stuff" theory 

 of thyroid substance. But the phrase "at a certain stage" 

 implies, as Allen's paper as a whole conclusively shows, that 

 the exact opposite of such a conception is alone tenable. 

 What actually happens, Allen brings to light, is that in 

 spite of the complete absence of thyroid substance trans- 

 formation of the larva is begun but is not carried through. 

 That is, the organs and parts of the adult frog are laid 

 down but (with the exception of the reproductive glands) 

 are not completed. "It is evident," Allen says, "that the 

 thyroid gland is in no wise essential to the earlier phases of 

 development, but that at a certain definite stage, further 

 development of the soma is dependent upon it." 7 It should 

 be mentioned that Allen makes a rather special point of the 

 accordance of his results with those of Gudernatsch. 



If the greatly hastened and modified metamorphosis of the 

 frog tadpole observed by Gudernatsch is inducible by no 

 other means than by thyroid substance (which while pos- 

 sible is not at all certain), then is the substance causal in 

 the important sense of being not only competent but in- 

 dispensable. But even so it falls far short of being a com- 

 plete causal explanation of the phenomena under contempla- 



