Metamorphosis 



compensated for by reimplantation of the 

 gland at another site. The animals then 

 metamorphose at the same time as the con- 

 trols (Allen, '29b; Etkin, '39). Such trans- 

 planted glands, since they are transplanted 

 as undifferentiated rudiments, must exert 

 their effects by virtue of their functional 

 development rather than by release of any 

 stored hormone they may contain. Thyroids 

 from tadpoles in late metamorphosis induce 

 precocious metamorphosis, whereas the 

 glands of premetamorphic animals are not 

 effective in this way (Swingle, '23; Slowi- 

 kowska, '23). Presumably these results are 

 produced by the release of stored hormone 

 from the larger glands and are therefore 

 comparable to the results of injection of 

 hormone rather than to transplantation of 

 primordia. 



To complete the evidence on the relation 

 of thyroid hormone to metamorphosis it 

 would be desirable to know what the level 

 of thyroid hormone in the blood is dviring 

 the normal metamorphosis. Unfortunately 

 evidence on this point is meager. Blood 

 plasma from metamorphosing axolotls and 

 tadpoles has been reported to induce meta- 

 morphosis in premetamorphic animals (Bal- 

 thasart, '31), but the data here are not 

 adequate. In the experience of the present 

 author, the feeding or injection of small 

 quantities of blood or tissues from tadpoles 

 in metamorphosis failed to induce metamor- 

 phosis in recipient tadpoles. 



The lack of evidence on the level of 

 thyroid hormone in the blood of amphibians 

 in relation to metamorphosis is compensated 

 for to some extent by histological studies 

 which show clearly that the secretory activ- 

 ity of the gland correlates closely with the 

 rate of metamorphic change. In the thyroid 

 gland the correlation between the cytological 

 and histological picture and the functional 

 activity of the gland is exceptionally clear. 

 A high level of activity is indicated by 

 columnar epithelium, mild basophilia, vac- 

 uolization and loss (release) of the colloid. 

 The criteria of activity have been studied 

 in particular detail by Uhlenhuth ('27) in 

 living and fixed thyroids of salamanders. 

 (For the general literature on microscopic 

 anatomy of the thyroid in relation to func- 

 tion see Means, '48.) 



Microscopic studies reveal a histological 

 picture indicating that the thyroid is mark- 

 edly more active at the time of metamor- 

 phosis than before. In urodeles colloid release 

 is a most prominent feature (Uhlenhuth, 

 '27); in anurans increased height of the 



633 



epithelium is more conspicuous (Etkin, '30). 

 Quantitative studies by Etkin ('36a) and 

 Mazzeschi ('40) show that at the beginning 

 of anuran metamorphosis the thyroid is 

 stimulated to grow much more rapidly than 

 the body as a whole. The cytological picture 

 likewise indicates increasing activity be- 

 ginning at this time, reaching a maximum 

 at the climax of metamorphosis and quickly 

 dropping back thereafter. These studies sug- 

 gest that the thyroid releases large quan- 

 tities of hormone at the time of metamor- 

 phosis and is relatively inactive before and 

 after. 



Fig. 218. Sagittal section of frog embryo at late 

 tail-bud stage. The thyroid primordium is removed 

 through slit made along broken line. (After Allen, 

 '18.) 



Another source of evidence pointing to 

 the dependence of metamorphosis upon the 

 thyroid hormone is afforded by the action 

 of thiouracil and related compounds. These 

 inhibit the release of thyroid hormone from 

 the gland. When administered to tadpoles 

 they have been found to inhibit metamor- 

 phosis (Gordon, Goldsmith and Charipper, 

 '43; Lynn, '48). 



Less direct but confirmatory evidence of 

 the essential role of the thyroid in amphibian 

 metamorphosis comes from the study of am- 

 phibians with exceptional life histories. In 

 the Jamaican tree toad, Eleutherodactylus 

 nubicola, no free-living aquatic larva devel- 

 ops but metamorphosis occurs precociously 

 within the egg membranes. The thyroids 

 likewise show histological evidence of pre- 

 cocious activation correlating well with 

 metamorphosis (Lynn, '36). In a similarly 

 precocious African toad, Arthroleptella bi- 

 color villiersi, metamorphosis is closely par- 

 alleled by thyroid activation (Brinck, '39). 



In permanently neotenous species of am- 

 phibians, as will be seen later, the failure to 



