196 



THE AMEBIC AN NATURALIST [Vol. LV 



It is remarkable that the administration of the same 

 amount of iodothyrine causes metamorphosis of sala- 

 mander larvR^ (){' different species in nearly the same in- 

 terval of tinir. Thus, (i.l ^m. iodothyrine per 1,000 c.c. 

 of water caused nictainoi phosis of A. opactim larvae in 7 

 days, of A. maculatuni larvje in 13 days and of A. tigrinum 

 larvae in 13 days. The time required to induce metamor- 

 phosis in thyroid-fed tadpoles decreases with increasing 

 age of the tadpoles. Gudernatsch (1 ) found that thyroid 

 feeding caused metamorphosis in I'O days if tadpoles of 

 a certain age were employed, in il d.ix if l;i(l])()les 7' days 

 older than the first lot were eni})l()> cd. and in only 4 days 

 if the tadpoles were 14 days older than the lirst lot. 



As pointed out above, the larva', when fed thyroid sub- 

 stance, may undergo the most rcnia I'kahlc (h'velopment, 

 although no growth may take place This seems to be of 

 great importance in many wa\ s. In all organisms de- 

 velopment and growth, nnder normal conditions, proceed 

 in a parallel way. The l)eha\ ior of the thyroid-fed larvae 

 suggests that the reason why no development takes place 

 without growth is the fact that, under normal circum- 

 stances, the substances which cause development of cer- 

 tain organs are supplied through the same reactions 

 which control the growth of the organism. If these sub- 

 stances are supplied to the organism from without, 

 development may |)roceed at a higher rate than growth 

 or may ])roce(Ml even in \\w coni])letc absence of growth 

 and thu^ the relation hr\^^rru urowth ami development 

 mav l.,Tonie chanuvd as in the thyroid-fed larvie. 



I'he chan-o of the r.'lation \u■\^^r,■u uroutli and de- 

 velopment furnish an ini[M.r'tant link in the chain of facts 

 that we must know in ordei- to nmh'istand the mechan- 

 ism of the thyroid appai-atus as well as that of the am- 

 phibian metamorphosis. Althonuh nmlei- cei-tain condi- 

 tions growth may be inhihit<Ml completely nixni the 

 feeding of th\ roid. tlii^ i^ no' al\\a\^ theca^.'. ['.oth tlu' 

 rate of develoi)ment and the rate of growth ai-e dei)endent 



