312 INFLUENCE OF THYMUS 



The question arises now in which way do some of the thymus-f ed 

 larvae procure enough of the substance required for the development of 

 the thyroid, while other thymus-f ed larvae are unable to obtain 

 enough of it though all of these larvcc apparently are fed on an 

 equally exclusive thymus diet? Although at present an exact state- 

 ment on this matter is impossible we must seek its explanation in the 

 fact that evidently the amount required is so small that it was diffi- 

 cult in our experiments so far performed to control the sources which 

 sometimes supplied this substance against our will. As to the actual 

 source of the substance it is possible that the connective tissue con- 

 stituting the septa between the lobules of the thymus may contain 

 some of it; in fact, this is probable since the septa are in no way a 

 specific tissue like the rest of the thymus. The possibility of some 

 of the larvae having obtained, by chance, more of the septa than others 

 must be admitted. In this way the great variability of the results 

 of the same author could be explained. There is also a possibility 

 that the water may contain some of that substance; in favor of this 

 would be the fact that some authors, like Swingle,^ did not obtain any 

 retarding effects at all in their thymus-f ed series. Of course, the dif- 

 ference in the reaction of different individuals of the same series of 

 one experimenter can be explained less readily on that possibility. 



Our experiments suggest the possible character of the influence of 

 environmental factors on metamorphosis. The problem of amphibian 

 metamorphosis, as well as the problem of internal secretion, assumes 

 a new shape in the light of that fact. On the one hand, it has become 

 clear from the experiments of Allen and his followers that meta- 

 morphosis is directly dependent on the action of a certain inner 

 secretory gland of the amphibian larva; on the other hand, it is evi- 

 dent that the development of the secretory stage of that inner secre- 

 tory gland depends ultimately on certain purely environmental, 

 non-glandular factors. And it is now time that we should recall 

 such attempts as those made by DumeriP and von Chauvin^" to en- 

 force or prevent metamorphosis by purely external non-glandular 



8 Swingle, W. W., /. Exp. ZooL, 1917-18, xxiv, 521. 

 ^ Dumeril, A., Ann. sc. nat., ZooL, 1867, vii, 229. 

 ^^ von Chauvin, M., Z. wiss. ZooL, 1885, xli, 365. 



