Huxley: Glands and Development 



357 



ABC, curve of general growth of 

 larvae; Ti-T4, curve of relative thyroid 

 growth in bullfrog, leQpard frog, Rana 

 pipicns, Rana tcmporaria, and Bufo 

 lentigularis respectively. 1, 2, 3, 4, 

 points where metamorphosis occurs ; 4a, 

 ^h, metamorphic points when the growth 

 in T4 is accelerated by weak and strong 

 iodine treatment respectively. 



Time 



FACTORS DETERMINING TIME OF METAMORPHOSIS 



Figure ii. Metamorphosis may be conceived of as being brought about by differential 

 growth of the thyroid and the body tissues. At first the body tissues develop faster than 

 the thyroid, but later on their rate of development slows down while that of the thyroid 

 increases. Inevitably, then, the curve for body growth and the curve for thyroid growth 

 will cross, and at this point metamorphosis occurs. Metamorphosis, like sex in Gold- 

 schmidt's moths, is an affair of balance, but the changes are irreversible and a decrease 

 in thyroid concentration after metamorphosis has taken place does not result in a return 

 to the larva state, or "katamorphosis" as it might be called. 



moths; alteration of temperature 

 causes alteration of degree of inter- 

 sexuality,— in other words, alteration 

 of the stage of differentiation at which 

 the "sex-metamorphosis" occurs. 



This matter of differential tempera- 

 ture-coefficient is obviously of im- 

 portance. What complex effects it 

 m.ay produce in what at first sight ap- 

 pears a simple phenomenon is shown 

 by such work as that of Fenn on 

 phagocytosis.' 



It is worth noting that this differ- 

 ential temperature-coefficient possibly 

 exerts an indirect effect upon sex in 

 Amphibia. Witschi found that tem- 

 perature changes brought about alter- 

 ations in sex-ratio in frogs ;^ and 

 Adler found that in late-fertilized 

 frogs (which, as is well known after 

 the researches of R. Hertwig's pupil. 



Kuschakewitsch, all become males"), 

 the thyroid was abnormally large\ 

 This, together with the previous con- 

 nection between temperature and rela- 

 tive rate of thyroid-differentiation, as 

 shown by alterations of metamorphic 

 size, makes it at least possible that 

 the sex effect of temperature in frogs 

 is connected with an effect upon the 

 thyroid. A good example of the rela- 

 tk'c nature of the thyroid's action is 

 afforded by the fact that the time re- 

 quired for metamorphosis by thyroid' 

 feeding in sexually mature Axolotls 

 is longer than in immature speci- 

 mens". This is also true for pituitary 

 metamorphosis'\ Apparently the change 

 of metabolism due to maturity, and in- 

 creased size and age raises what we 

 may metaphorically call the resistance 

 of the larval tissues to metamorphosis. 



