INFLUENCE OF THE THYROID 



GLAND AND HYPOPHYSIS UPON 



GROWTH AND DIFFERENTIATION 



Bennett M. Allen 

 Department of Zoology, University of Kansas 



RECENT years have witnessed spec- 

 tacular advances in our knowl- 

 edge of the endocrine or ductless 

 glands. This held was a terra incog- 

 nita thirty >ears or more ago, but 

 toda\' it is a field of enchanting possi- 

 bilities. The th\roi(l gland and the 

 Inpophysis, with which this paper 

 deals, exert a profomid influence upon 

 growth and differentiation, not only 

 among the amphibians upon which our 

 experiments have been performed, but 

 likewise in man as we are learning 

 from a stiuK- of dwarfs and giants. 



While clinical e\idence is of \ery 

 great importance in bringing us close 

 to the jjractical problems of medicine, 

 we arc here dealing with the influence 

 that these glands exert as a result of 

 disease, and we must constantly keep 

 in mind the fact that the conditions 

 observed are pathological, that the 

 endocrine glands are in these cases 

 fimctioiu"ng in abnormal ways. With 

 ex|)erimi-nts upon maiumals we meet 

 the (liHi(ult\- that these glands can be 

 removed only after birth, while as a 

 matter of fact, the embr>o undergoes 

 the most important part of its develop- 

 ment before birth. Definite direction 

 is already given to some of the most 

 important features of growth and 

 ditlcrenti.itiou of manuuals long before 

 we can hope to operate upon (hem. We 

 have no intention of belittling the very 

 significant pioneiT work, such as Hoff- 

 meister and others, practiced in the 

 removal of the th>roid glands of dogs 

 and .sheep. Such work has its great 

 value but it does not give a wholly 

 pure result. We are not in the.se cases 

 dealing with animals that never have 

 been under the intluenci- of these 

 glands. Whili- these various methods 

 of experimentation have their value, we 

 can gel a "chemically pure" reaction 



only when we may remove these glands 

 at their inception when we extirpate 

 their anlagen. By far the most favor- 

 able o|)portunit\' for work of this kind 

 is afforded b\' ex|X"riments upon am- 

 phibian lar\ae, which develop from 

 eggs fertilized outside the body of the 

 parent. The tadpoles are peculiarly 

 resistant to the most severe operations, 

 showing a marvellous capacity for heal- 

 ing wounds and there is an unusual 

 degree of resistance to infection. The 

 process of differentiation is marked by 

 the spectacular metamorphosis of the 

 tadpole into the frog. These favor- 

 able features make it possible for us to 

 remo\e these glands with success and 

 to observe the effects most clearh-. 



We may also attack these problems 

 from a different angle — by adding these 

 glandular secretions instead of elimi- 

 nating them. This may be done in 

 three ways: (1) by feeding, (2) by 

 hypodermic injection, and (3) by im- 

 plantation. By a combination of proc- 

 esses of gland removal and addition of 

 extracts and living tissue, we may at 

 will create in our tadpoles almost any 

 desired combination of endocrine 

 glands. We ma>- remove and reinstate 

 at pleasure. The thyroid gland was 

 fed to tadpoles l)y C.udernat.sch in his 

 pioneer investigations. This experi- 

 luent man>' times repeated 1)>- others 

 results in precocious metamorphosis— 

 a process that affects the organism as a 

 whole, not only involving the develop- 

 ment of the limbs, tongue, mouth, 

 brain, and other internal structures, 

 but bringing about the shortening of 

 the intestine and disappearance of the 

 tail as well. 



A complete reciprocal of this experi- 

 ment is found in the removal of the 

 thyroid gland. This exjieriment was 

 performed simultaneously by Allen and 



414 



