ESSAYS 305 



Amphibian metamorphosis is due to the interaction of different environ- 

 mental agencies, such as the iodine content of the medium, with the genetic 

 factors controlling growth. This opinion has been warmly supported by 

 Uhlenhuth, whose work has dealt chiefly with salamanders. Uhlenhuth 

 (19 19) finds that the shedding of gills and larval skin are synchronous, and 

 depend upon the presence of iodine : by varying conditions of temperature, 

 the assumption of adult coloration and the onset of sexual maturity can 

 be produced before or after the actual metamorphosis, while the develop- 

 ment of tongue and palatal teeth may be checked altogether. He deduces 

 inconclusively that different chemical factors are involved in the develop- 

 ment of different series of larval organs ; indeed, both Swingle and Uhlenhuth 

 appear to entertain the idea that such factors are independently related 

 to the environment, and not directly co-ordinated by the organism as a 

 whole. But the evidence does not necessarily imply either : adopting the 

 hypothesis of specific activators in somatic differentiation, it is plausible to 

 think of them as individual products of a continuous chain of processes : 

 the amount of any one available at a given moment, and the effect produced 

 in consequence, depend on mass action conditioned by the efiect of tempera- 

 ture, among other things, upon the relative velocities of the component 

 reactions. How far such factors are interrelated or independently derived 

 from the environment or the activities of internal organs cannot at present 

 be decided without exceeding the limits of legitimate inference. 



In this connection it is instructive to note that apparently similar and 

 normal metamorphic changes can be induced by methods which are not 

 easy at present to correlate with Swingle's results. D. I. Macht (1919) 

 has recently claimed that frogs undergo metamorphosis precociously with 

 prostate feeding. Possibly, in this case, the effect is produced by causing 

 the thyroid to discharge its contents into the vascular system. However 

 that may be, more serious difficulties are raised by the case of the normally 

 neotenous Mexican salamander. Marie de Chauvin and Boulenger's ex- 

 periments clearly demonstrate that metamorphosis can be enforced in 

 Axolotls, that would not otherwise have transformed, if they are forced to 

 breathe air. That this has any direct connection with thyroid function 

 there is no evidence whatever. Laufberger (1913) and Jensen (1917) suc- 

 ceeded in enforcing the metamorphosis of large Axolotls by thyroid feeding, 

 and this result has been recently confirmed by Huxley (1920) and Hogben, 

 who have found iodine also instrumental in producing metamorphic changes. 

 In such animals no deviation from the normal type of metamorphosis has 

 been observed as the result of thyroid feeding. 



§3 

 Side by side with such investigations as those referred to, attention 

 has naturally been directed to the influence of other ductless glands upon 

 metamorphosis in the Amphibia. Concerning the pituitary, there is little 

 matter of interest to record. On the other hand, the researches of Bennet 

 Allen and Uhlenhuth into the activities of the thymus and parathyroids are 

 interesting, and reopen the question of the reputed endocrine function of the 

 former. The author first named confirms Hammar's earlier work, denying 

 that the Cixtirpation of the thymus affects metamorphosis or has any influ- 

 ence on growth and internal differentiation of Bufo larvae. In accordance 

 with this work, Uhlenhuth submits evidence that the inhibitory efiects de- 

 scribed by other authors as accompanying thymus feeding are due to a 

 deficiency of diet, which, appropriately balanced, can be rectified so as to 

 give completely negative results. Such larvae, fed on thymus, frequently 

 exhibit tetanic symptoms, when they reach the stage at which that organ 

 is fully developed : addition of sufficient ordinary diet to induce meta- 

 morphosis does not remove the symptoms ; but they cease to appear at the 



