358 



The Journal of Heredity 



The reason that the thyroid of frog 

 larvae belonging to large species 

 which do not metamorphose their first 

 year may yet be active when grafted 

 into larvae of other species need not, 

 then, mean that the grafting has in 

 some way altered their tunction, or 

 that an "excretor substance" is pres- 

 ent in the grafted larvae which stimu- 

 lates the grafts to secrete outwards 

 instead of to store their secretions. 

 It merely indicates that the amount 

 of thyroid secretion in larvae of the 

 slow-metamorphosing species remains 

 relatively too low for a longer time 

 than in the others. It is important to 

 notice that this relative slowness in the 

 amount of thyroid secretion may be 

 brought about in two opposite ways — - 

 first by decreasing the relative rate 

 of differentiation of the thyroid, or 

 secondly by altering the activity of 

 those other processes in the body 

 against which the thyroid acts. 



Whenever a phenomenon is the re- 

 sult of two opposing tendencies, this 

 double possibility of altering the re- 

 sult will of course be present. For 



instance, it is well known that the re- 

 sorption of a malignant tumor may 

 be due to damage inflicted on the 

 tumor or to raising the resistance or 

 general activity of the rest of the 

 organism. In a similar way recent 

 physiology emphasizes the existence 

 of a similar balanced action in certain 

 neuroses."" 



If we look at our facts in the 

 light of our analogy from sex, we 

 may say that metamorphosis is com- 

 parable to consecutive hermaphrodit- 

 ism, and is thus a normal process 

 similar to the abnormal process caus- 

 ing intersexuality in the Gypsy Moth. 

 It is consecutive dimorphism ; and is 

 due to the fact that the hereditary con- 

 stitution of most Amphibia is so con- 

 structed that a thyroid is produced 

 only after a certain stage, that it then 

 differentiates relatively more rapidly 

 than the rest of the organism, and 

 that the relative amount of thyroid 

 secretion therefore gradually increases 

 until it reaches the threshold value 

 necessary to bring about metamor- 

 phosis. (Fig. II.) 



{To he contirmcd.) 



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