The Journal of Heredity 



adult tissues at a higher rate of meta- 

 boHsm. and that when the metabohc 

 level is raised beyond a certain point, 

 the larval tissues break down. That 

 tissues are unable to maintain them- 

 selves in a functional state when cer- 

 tain alterations in conditions occur, — 

 in other words, that they dcdiffercn- 

 fjaf(, — is well known. When all tissues 

 dedifferentiate mors or less simultan- 

 eously, there is a regression of the 

 whole animal to a simplified state, as 

 in Clavellina. When only some are af- 

 fected, the result may be a complete 

 resorption of the afifected organs, due 

 to migration of the cells into the cavi- 

 ties of the body;'* or a beginning of 

 resorption followed by phagocytosis. It 

 is this last method which appears to 

 occur in Amphibian metamorphosis. By 

 starving young tadpoles, I have been 

 able to obtain localized dedififerentiation 

 in small areas of the tail, the histologi- 

 cal appearance being very like that 

 which obtains at metamorphosis. In 

 this case, phagocytosis only appears to 

 ol)tain to a very small extent. 



The Mechanism of Metamorphosis 



The important researches of Champy 

 have thrown a new light on the 

 threshold values for different tissues. 

 He finds that in frog tadpoles, after 

 a single administration of thyroid, cer- 

 tain tissues respond by a progressive 

 increase in the proportion of mitoses, 

 others by a progressive decrease, while 

 still others show no change in growth- 

 rate. What is more, one and the same 

 tissue may show localized areas of all 

 three descriptions, distinguishable solely 

 by their mode of reaction to thyroid- 

 diet. In other words, the action of 

 thyroxin, like that of so many other 

 hormones, depends not only on its own 

 specific properties, but also on specific 

 properties of the tissues it afTects. As 

 we have said before, there are there- 

 fore two classes of variables to be 

 kept in mind in dealing with any 

 ])roblem such as metamorphosis, into 

 which the thyroid enters, variations in 



thxroxin concentration, and variations 

 in the sensitiveness of the tissues. 



If this is so, the primary mechanism 

 of metamorphosis should be (a) an ad- 

 verse effect upon the purely larval tis- 

 sues, followed by their partial dediffer- 

 entiation. this followed by incipient re- 

 sorption, and this by phagocytosis ; (b) 

 an acceleration in the rate of growth 

 of all specifically adult tissues, which 

 are those that are adapted to grow bet- 

 ter at a higher rate of metabolism. 



W^e have in this a general answer 

 to our second principal question, as 

 to the mechanism of metamorphic 

 change. To pursue this point in detail 

 would take us too far afield, and we 

 must content ourselves with the very 

 general statement that certain tissues 

 have a lower optimum metabolic rate 

 for maximum growth, others a higher, 

 and that the former spontaneously 

 break down v/hen the rate is raised too 

 far. their debris being cleared away 

 later by the phagocytes. 



Aletamorphosed Amphibia, so far as 

 known, show no important morphologi- 

 cal changes when their thyroids are re- 

 moved. If this is corroborated, it 

 shows that the high concentration of 

 thyroid is not necessary for the mainte- 

 nance of adult structure in the same 

 way that low concentration is necessary 

 for the maintenance of larval structure, 

 and emphasizes the irreversibility of 

 metamorphic processes. 



Bearing in mind the idea of the two 

 internal environments, distinguished by 

 dift'erent rates of metabolism, we may 

 be able to grasp certain other of our 

 problems more clearly. 



In parenthesis, it may be observed 

 that some recent writers, e. g.. Swin- 

 gle'" and Kendall," believe that the 

 thyroid secretion acts in different ways 

 in' Amphibia and in ]\Iammals, the 

 metamorphic effect in the former be- 

 ing due solely to the iodine content of 

 the thyroid hormone, the rise in basal 

 metabolism seen in the latter being due 

 to a specific iodized organic compound, 



*For numbered references, see "Literature Cited" at end of article. 



