650 



Metamorphosis 



toma metamorphose with the latter, but in 

 the reverse transplant, i.e., from Amblystoma 

 to axolotl, the transplant remains indefinitely- 

 larval (Shtern, '33). The well known arti- 

 ficial induction of metamorphosis in the 

 axolotl by thyroid and pituitary treatments 

 likewise points to the existence of tissue sen- 

 sitivity, although not indicating the level of 

 sensitivity. It may be inferred that the neot- 

 eny of the axolotl results from lack of glan- 

 dular activity, that of perennibranchs from 

 loss of tissue sensitivity. 



Insects. In insects as in amphibians each 

 structure of an animal responds in its own 

 specific way to the common metamorphic 

 stimulus. Also with regard to the early time 

 of acquisition of tissue specificity there is a 

 striking similarity between amphibian tis- 

 sues and those in insects (except possibly for 

 Diptera, as noted below). Piepho ('38), for 

 example, finds the capacity to pupate appears 

 in caterpillars upon emergence from the egg. 

 Similarly in Rhodnius the ability of early 

 instars to respond to the metamorphic stimu- 

 lus seems substantially complete (Wiggles- 

 worth, '40). Bodenstein ('35) found that the 

 stage from which the thoracic leg transplant 

 was taken made no difference in its respon- 

 siveness to host stimuli in Vanessa. Al- 

 though no quantitative evaluation of the sen- 

 sitivity of the tissues seems to have been at- 

 tempted, it can be seen from the facts stated 

 that in these forms, at least, changes in tis- 

 sue sensitivity do not play a significant role 

 in determining the time or character of molt 

 or metamorphosis. Scharrer ('52a) has inter- 

 preted the production of incomplete adults 

 after C.A. removal at early instars in Leuco- 

 phaea as evidence that the tissues in these 

 animals are not yet fully competent to give 

 the complete hormone response. However, 

 such effects may merely result from the per- 

 sistence of C.A. hormone as currently under- 

 stood. 



In Drosophila, on the other hand, there is 

 evidence that a progressive increase in re- 

 sponsiveness of the tissues takes place during 

 larval development. Hadorn and Neel ('38) 

 found older larval tissues more sensitive than 

 younger ones to the pupation stimulus from 

 ring gland implants. Bodenstein ('43) re- 

 ported greater response to transplanted ring 

 glands from older imaginal discs, and similar 

 results were reported by Vogt ('42). It would 

 appear from these results that changes in 

 tissue responsiveness may play a significant 

 role in insect development. On the other 

 hand, the greater response of older discs may 

 come about because they start from a more 



advanced base line rather than because of 

 any increase in sensitivity. This interpreta- 

 tion seems more consistent with the view, 

 developed above, that larval development in 

 Diptera as in other insects is controlled by 

 a succession of hormonal pulses. Sensitivity in 

 insects, as in amphibians, may be acquired 

 fully at an early stage. 



METAMORPHIC PATTERN 



Amphibians. The importance in metamor- 

 phosis of the time relationships of the 

 changes which take place is illustrated by 

 the abnormality of the metamorphic pattern 

 induced by strong doses of thyroid (Fig. 227). 

 The animals produced by such treatment 

 usually die in the process of metamorphosis 

 in a condition characterized by protruding, 

 overdeveloped lower jaws, with tails largely 

 resorbed but hind legs little more than un- 

 differentiated nubbins. Large open skin win- 

 dows for the forelegs may form but the legs 

 are not sufficiently developed to protrude. 

 Such distortions were early recognized and 

 descriptively analyzed a number of times 

 (Schreiber, '34a). Many authors speculated 

 on the possibility that the normal pattern 

 of metamorphic change results from differ- 

 ences in the threshold of response of different 

 tissues to thyroid hormone (see, for example, 

 J. Huxley, '22; Schreiber, '34b). To test such 

 theories a number of workers have studied 

 the effects of immersion in different concen- 

 trations of thyroid substances upon the 

 progress of metamorphosis in tadpoles (Allen, 

 '32b; Etkin, '35a). Since the results of these 

 studies, consistent among themselves, have 

 repeatedly been misinterpreted as supporting 

 the idea that differential thresholds of re- 

 sponse account for the spacing of metamor- 

 phic events (for example, see Needham, 

 '42), it is necessary here to examine the idea 

 of thresholds with some care. 



The technique used by Blacher and Allen 

 was to observe the tadpoles after a definite 

 period of immersion in solutions of different 

 concentrations of thyroid substance and note 

 which metamorphic changes had occurred. 

 This procedure showed that events charac- 

 teristic of the early stages of metamorphosis, 

 such as leg growth and intestinal reduction, 

 occurred within the specified period at lower 

 concentrations of the thyroid hormone than 

 did the later metamorphic events, such as 

 mouth changes and forelimb emergence. It 

 is clear, however, that this procedure does 

 not reveal true thresholds, for it is possible 

 that by observing the animals for longer 



