304 SCIENCE PROGRESS 



month. Subsequent investigation by the same worker estabhshed that 

 similar results could be obtained by the use of iodine free of organic com- 

 bination, and that, administered in dilute solution added to the culture or 

 in elemental form mixed with the food, iodine itself is more effective for 

 this purpose than any of its compounds. 



Such experiments, although suggestive, do not of themselves yield 

 critical data respecting the functional activity of the thyroid gland in the 

 organism to which it belongs. A basis for more extensive inquiry was 

 afforded by Bennet Allen's success (1918) in perfecting the technique of 

 thyroidectomy in very young tadpoles, followed by similar work on the 

 part of Terry and E. R. and M. M. Hoskins. These investigators have 

 demonstrated the complete inhibition of metamorphosis consequent upon 

 the removal of the gland. And it appears that, while total absence of the 

 thyroid results in a cessation of somatic differentiation, it is not found to 

 hinder continued growth in size or to retard the development of the gonads. 

 Swingle has made use of Allen's material to obtain further knowledge of the 

 modus operandi of the thyroid diet method of enforcing metamorphosis 

 prematurely. By feeding thyroidless tadpoles of Bufo with a diet contain- 

 ing iodine crystals, he finds that they are capable of completing the process 

 of somatic differentiation. Histological examination, moreover, according 

 to Swingle, reveals that in normal tadpoles fed with iodine the glands are 

 of greater size and the follicular colloidal masses enlarged. 



It is possible to distinguish at least two types of endocrine organs : in 

 the first place, such an organ may function by simply setting free hormones 

 secreted by its own activity, as in the case of the suprarenal medulla ; 

 secondly, in considering the controlling influence of the gonads on secondary 

 sex characters, Geoffrey Smith's work indicates that there is another type 

 of hormone-producing organ, one which exercises its influence in a less 

 direct and autonomous manner by causing, through its own metabolic 

 activities, the production elsewhere of those substances which supply the 

 characteristic stimulus. In the first case, feeding with the gland or its 

 active principle proves an effective antidote for absence or removal ; and 

 it is usually assumed that the thyroid conforms to this category. Swingle's 

 experiments lead to the conclusion that, as far as the thyroid is concerned 

 with Amphibian metamorphosis, it does not function strictly in either of the 

 senses defined above. The relevant facts are — (i) that the thyroid normally 

 contains iodine, and that its iodine content can be increased by including 

 iodine or its compounds in the diet ; (ii) that the administration of iodine 

 produces a precocious metamorphosis in normal larvae ; and (iii) that with- 

 out the administration of iodine thyroidless larvae are incapable of under- 

 going a normal metamorphosis. These data, considered collectively, point 

 very strongly to Swingle's conclusion that, in relation to metamorphosis, the 

 thyroid functions as a storage organ, extracting iodine from the blood and 

 retaining it for future use. 



§ 2 

 In accepting this view, it is essential to appreciate clearly that the term 

 "metamorphosis " is used by these authors in a more restricted sense than its 

 customary connotation, to apply specifically to the loss of such larval fea- 

 tures as the gills, tail or tail fin. Swingle, at an early stage in his experi- 

 ments, realised that the development of all the larval organs is not accelerated 

 to the same extent by the methods he employed ; in particular, the gonads 

 are unaffected by the procedure of thyroid or iodine feeding. Entirely in 

 unison with his observations are those of Bennet Allen (1918) and of E. R. 

 and M. M. Hoskins (19 19), who were able to rear abnormally large and 

 neotenous (i.e. with sexually mature gonads) tadpoles in their thyroidec- 

 tomised cultures. Swingle has also recorded the relatively shorter length 

 of the gut in his precocious frogs ; and he advocates the hypothesis that 



