Xo. G3S] IXTEBXAL SECHETIOXS OF AMPHIBIAXS 197 



Oil the quantity of thyroid substance administered to the 

 larvae. Up to a certain quantity, growth as well as de- 

 velopment is accelerated; if the (luantity administered is 

 further increased, growth hecdincs more and moi'c in- 

 hibited, while differentiation is iiK-rcasiiiuly ac-cflciatrd. 

 With very large doses the thyi-oid sul),staiicc may .'ftVct 

 even a decrease in the size and wci.uiii of the lai'va- while 

 development of the limbs is ^ivatly accch'rntcd in the 

 beginning, it finally stops ami the animals die from 

 emaciation (6). 



Kendall (7) has shown that in man the tliyroid iKo inoiie 

 increases the basal metabolism in a strictly t|iiaiititative 

 way. Determinations of metabolism have not hwn made 

 in amphibians, but the behavior of the thyroid-fed tad- 

 poles as described above indicates that the thyroid hor- 

 mone also increases highly the metabolism of the cold- 

 blooded organism. If too much of tlic liormonc is 

 administred, metabolism is increased in sudi a niaiiiier 

 that catabolism becomes higher than anal)olisiii. since 

 the organism no longer is capable of supplx inu enough 

 food materials from outside io maintain a positive meta- 

 bolic balance, and eonse.iuent ly the body -nl.stanee it>elf 

 is broken down and a decrease in size and luxly weiuht 

 takes place. Finally even development hee.mu's impos- 

 sible. For tli!> I'eason, as Lenhart (<i) .-howed. more 

 thyroid substance ean be administered without lea.ling 

 to a check of development if tlu' thyroid-fed lai'va' are 

 either kept under conditions which decrease metaltolism, 



(carbohydrates) which can l)e made easily available for 

 metalxdism. 



From these facts it seem> evident that the amphibian 

 metamorphosis is the result of a lii-hly inen'a.ed metab- 

 olism, or more c<.rrectlv, metamorplio>i> seem^ to result 

 if nn'taboli>ni is inereased in sueh a degree and manner 

 that eataboliMH becomes higher than anabolism. The 

 question arises whether substances or agents other than 



