DISCOVERY 



149 



are fed with thyroid tissue. This discovery of Allen's 

 was soon afterwards confirmed by the work of E. R. 

 and M. M. Hoskins. And we may say at this point 

 that the behaviour of the thyroidless tadpoles completes 

 the evidence for regarding the activity of the thyroid 

 gland as an essential factor in controlhng the meta- 

 morphosis of the frog and its allies. 



This fact is of considerable interest to those who are 

 concerned primarily with the physiology of develop- 

 ment and growth. But the metamorphosis of the 

 tadpole has an ulterior significance which is related to 

 important questions in human physiology. That the 

 thyroid is a ductless gland was first estabhshed by the 

 discoverv that the condition of arrested physical and 

 mental growth (cretinism) in children associated with 

 thyroid deficiency is remediable by injection, implanta- 

 tion, or feeding with thyroid gland substance. For 

 many years it has been known that the thyroid con- 

 tains a higher percentage of the element iodine than 

 any other portion of the body ; and iodine compounds 

 have been administered medicinally in thyroid dis- 

 orders. The precise significance of the connection 

 between iodine and the thyroid has been the subject 

 of much controversy. Some have regarded the 

 presence of iodine in the thyroid as unimportant ; 

 others have held that the thyroid hormone (or 

 antacoid) is an iodine compound. It is, of course, 

 impossible to traverse the intricacies of this discussion 

 in this article. It is sufficient to say that the study 

 of the transformation of the tadpole into the frog has 

 heavily weighted the scales in favour of the latter view. 



Soon after Gudematsch's observations appeared 

 Morse succeeded in obtaining similar results with the 

 use of iodised blood albumen ; and Lenhart (1916) was 

 able to show that the rate of metamorphosis in tadpoles 

 fed on thiyToid diet depends on the iodine content of 

 the glands used. These observations led to a brilliant 

 series of inquiries by Swingle (1918-20), who proved 

 that iodine (free of organic combination) administered 

 either with the food or dissolved in the water wiU not 

 only induce, hke thyroid substance, a precocious meta- 

 morphosis in normal larvse, but will also enable 

 thyroidless tadpoles to complete their development. 

 Frog tadpoles can be made to complete their meta- 

 morphosis even when deprived of their thyroids, if a 

 sufficient quantity of iodine is supplied to them. 

 The same is true of salamander larvae (Uhlenhuth). 

 Thus in these creatures the thyroid acts as a storage 

 organ for iodine, accumulating the latter as the 

 immature larva grows from the minute traces of iodine 

 in their normal food, and eventually liberating it in 

 the form of a highly activ-e substance which, circulating 

 in the blood stream, leads to the destruction of the 

 larval characters (gills and tail) and to the growth of 

 the forehmbs and other adult features. 



It has been mentioned that the relation of the 



thyroid to metamorphosis has been estabhshed in 

 newts and salamanders as well as in frogs and toads. 

 These tailed amphibians also start their free life with 

 a bodily organisation very different from that of the 

 adult form ; but the details are not quite the same. 

 In newts and salamanders the metamorphosis is less 

 abrupt ; they are hatched from the egg with limbs fully 

 developed ; and they do not — like the tadpole — 

 possess a giU-cover of operculum. In its place there 

 arises on either side of the head, as in the very young 

 frog tadpole, a tuft of filaments, the external gills. 

 Metamorphosis essentially consists in the loss of these 

 giUs and the fin which runs along the back and either 



Fig. I- A. Axolotl larva. 



B. Intermediate stage after a fortnight's thyroid feeding. 



C. The fuUy developed Mexican Salamander. 



side of the tail, which latter passes over into the adult 

 and is not destroyed as in frogs and toads. Sometimes 

 these amphibian larvae fail in nature to transform at 

 the ordinary time and assume sexual maturity while 

 still in the larval state. This has become a permanent 

 condition in the case of the Mexican salamander of cer- 

 tain locahties. The " axolotl," or larva of the Mexican 

 salamander, never leaves the water, but reproduces 

 from generation to generation in the gilled condition. 



XaturaUy, when young specimens were imported 

 from Mexico, reared to maturitj-, and bred in the 

 Jardin des Plantes for the first time, in 1863, they 

 were regarded as representatives of a small tribe of 

 salamanders which retain the gills, etc., permanentlv 

 throughout life like the blind cave newt Proteus of 

 Dalmatia and the " hell bender " {Nectiirus) of North 

 American rivers. It was found a year later that 

 offspring of the same stock would, if the water dried 

 up, shed their giUs and tail fin, develop eyelids and 

 yellow spots on the belly, and generally assume the 

 appearance of a well-knowii American salamander of 

 terrestrial habit. The transformation can be brought 

 about by less drastic means and with infallible regularity 



