DUCTLESS GLANDS AND 

 DEVELOPMENT 



Amphibian Metamorphosis Considered As Consecutive Dimorphism, 

 Controlled by the Glands of Internal Secretion 



Julian Huxlkv 

 Oxford University. England 



A GREAT deal of work has been 

 done on the problem of Am- 

 phibian Metamorphosis since 

 the discovery of Gudernatsch'"* that a 

 thyroid diet caused precocious meta- 

 morphosis. It is not my purpose here 

 to summarize this work, but to try to 

 set out the problem in the most gen- 

 eral light possible, and to draw atten- 

 tion to parallels with other lines of 

 work which seem to promise fruitful 

 results. Especially I would like to 

 draw attention to Amphibian Meta- 

 morphosis as offering unequalled oppor- 

 tunities for studying developmental 

 physiology from a rather new angle. 



It is clear toda}^ that the thyroid 

 plays a predominant role at metamor- 

 phosis with the pituitary as junior 

 partner ; but further than this it has 

 been difficult to penetrate. There are 

 two main problems which have at- 

 tracted attention. The first may be 

 summed up in a question : Why does 

 the thyroid act at metamorphosis and 

 not before? Young tadpoles of Anura 

 long before metamorphosis have a 

 well-developed thyroid, as does the 

 facultatively neotenous Siren." Why 

 is it that the thyroid in these cases 

 does not produce its characteristic 

 effects? And how is it that in some 

 species of Anura metamorphosis 

 occurs within a few months, in others, 

 as in bull-frogs, only after two or 

 three years? 



The second problem is equally gen- 

 eral. Granted that the thyroid does 

 produce metamorphic effects, how does 

 it do so? Why does thyroid feeding 



not produce metamorphosis in perma- 

 nently neotenous species such as Nec- 

 turus?^*'-^ Why do certain tissues, 

 such as the gills and fins, react to 

 thyroid treatment by regression and 

 resorption, while others, such as the 

 legs of Anura, by active growth? 

 Why, further, do similar tissues, such 

 as the limbs, behave differently in 

 Anura and in Urodela? If a certain 

 amount of thyroid be necessary for 

 growth in human beings, as is shown 

 by the bad effects of congenital lack 

 of thyroid in cretins, how is it that 

 thyroidectomised tadpoles show a per- 

 fectly normal or indeed accelerated 

 growth rate? 



In regard to the second point, most 

 authors, if they discuss it at all, agree 

 that the changes in the larval organs 

 at metamorphosis are of a similar 

 kind to those seen in dedifferentiation 

 of whole animals (e. g. Clavellina) or 

 in involution of organs. Something 

 occurs which affects these particular 

 issues so that they can no longer re- 

 main functional. 



The first point has hardly received 

 the attention it deserves. The only 

 hypothesis, so far as I am aware, 

 which attempts to deal with the mat- 

 ter, is that which in various forms is 

 advanced by Uhlenhuth.22 Swingle^^ 

 and others. It is that during the larval 

 period the function of the thyroid is 

 purely one of storage, but that just 

 before the time of metamorphosis, a 

 change occurs in it, and it starts to 

 excrete its stored products, which only 

 then can affect the body and so bring 



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



349 



