646 



Metamorphosis 



mone. However, since the general nutritive 

 values or other positive or negative factors 

 carried in these fluids may play a role in the 

 reaction, they are not of themselves conclu- 

 sive. In spite of these lacks the concept of a 

 brain hormone and of a prothoracic hormone 

 seem fairly well justified. With respect to the 

 corpus allatum, the evidence for hormone 

 action is less direct and the possibility of 

 some action of the gland upon the metamor- 

 phosis hormone other than by secretion of 

 a separate principle has not been excluded. 

 As noted above, Wigglesworth has postulated 

 elimination of the C.A. hormone by this 

 gland in the pre-metamorphic instar. In any 

 case it seems important to realize that the 

 properties of the metamorphic principles in 

 insects may be somewhat different from 

 those we are accustomed to see in vertebrate 

 hormones. A notable instance is the slowness 

 with which the principles may spread from 

 one parabiont to another, as seen in the re- 

 ports of Wigglesworth ('34) and Williams 

 ('52; Fig. 226). Again the prominence of 

 local effects from C.A., effects that persist 

 through several molts (Piepho, '50) seems 

 hardly consistent with the idea that the 

 viable graft is secreting a hormone. The per- 

 sistence of C.A. effects for one or two molts 

 after removal of the corpus (Scharrer, '46; 

 Pflugf elder, '37) is also difficult to reconcile 

 with the usual prompt disappearance of hor- 

 mones from circiilation, at least as hormone 

 action is known in vertebrates. 



METABOLISM IN RELATION TO 

 METAMORPHOSIS 



The well-established effect of the thyroid 

 hormone in raising the rate of metabolism 

 in mammals (Means, '48) raises the prob- 

 lems of whether this hormone has a similar 

 effect in the amphibian larva and whether, if 

 it has, this effect is responsible for meta- 

 morphic changes. The possibility that a meta- 

 bolic effect underlies metamorphosis has pro- 

 vided the basis for numerous speculations on 

 the nature of metamorphic change (e.g., 

 Huxley, '29). Yet the weight of the experi- 

 mental evidence is against the concept of a 

 role of metabolic rate in metamorphosis. In 

 the first place, despite the evidence of earlier 

 studies it is dovibtful whether the thyroid 

 does exert a metabolism-increasing effect 

 upon amphibian larvae. The evidence up to 

 1934 was examined by Etkin ('34), who con- 

 cluded that the increase in oxygen consump- 

 tion per unit weight that has been reported 

 during thyroid-induced amphibian meta- 



morphosis did not signify an increase in 

 metabolic rate, since it resulted from the loss 

 of body water rather than from an increase 

 in oxygen consumption per individual or per 

 unit dry weight. In studies on the oxygen 

 consumption of tadpoles during normal meta- 

 morphosis in the bullfrog, Etkin fovmd no 

 increased oxygen consumption irrespective of 

 the basis of calculation. The rate per indi- 

 vidual animal decreased during the meta- 

 morphic climax. Furthermore, the analysis 

 of the literature on cold-blooded vertebrates 

 shows that there is no adequate evidence for 

 ascribing a metabolic rate effect to the thy- 

 roid hormone in cold-blooded vertebrates 

 generally (Etkin, Root and Mofshin, '40). 

 Though the evidence in this field continues 

 to be conflicting, subsequent reports have in 

 general confirmed the above interpretation 

 (Smith and Everett, '43). 



Even the workers who ascribe to thy- 

 roid a stimulating effect on metabolism dur- 

 ing amphibian metamorphosis do not regard 

 this effect as of causal significance in the 

 process of metamorphosis itself (Helff, '26; 

 Needham, '42). Dinitrophenols, which raise 

 oxygen consumption in mammals and fash 

 (Means, '48; Root and Etkin, '37), do not 

 influence amphibian metamorphosis (Cut- 

 ting and Tainter, '33). 



Cyanides have been reported to inhibit the 

 effect of thyroid on metamorphosis, and this 

 has been taken as supporting the metabolism 

 theory in metamorphosis (Demuth, '33; Hoff- 

 man, '35). Borland ('43), however, found a 

 marked accelerating effect of cyanides on 

 metamorphosis. In any case the evidence from 

 cyanides, like that from dinitrophenol, is too 

 indirect to be very helpful on the question of 

 the role of metabolic increase in metamor- 

 phosis. Nevertheless, taken as a whole the 

 weight of the evidence negates the idea of a 

 causal role of a metabolism-raising factor in 

 amphibian metamorphosis. 



An influence of the corpvis allatum on 

 metabolic rate in insects has been reported 

 by Thomsen ('49), who fovmd a 20 per cent 

 decrease after allectomy with almost com- 

 plete restoration of the rate by replacement 

 therapy. She regarded the activity of the hor- 

 mone as being primarily on metabolic rate. 

 The numerous influences exerted by the 

 C.A. on the physiology of the adult insect 

 (Pfeiffer, '45b) may also be cited as evidence 

 for a fundamental role of C.A. on basic meta- 

 bolic processes. 



Basal metabolism, being the summation of 

 numerous and varied metabolic pathways, is 

 too general a concept to yield significant data 



