648 



Metamorphosis 



weak spot or opening formed in the auto- 

 lyzed area. A number of mechanisms were 

 suggested to explain the formation of the 

 window. Weber ('25), for example, main- 

 tained it is produced by the action of the 

 poison glands at the base of the leg. The pres- 

 sure of the developing leg is an obvious pos- 

 sibility. But no experimental analysis was 

 offered until Helff ('26) showed that skin 

 from other areas transplanted to the appro- 

 priate place would undergo autolysis during 

 metamorphosis and, conversely, opercular 

 skin transplanted to the back would not form 

 the opening. This was interpreted as showing 

 that the skin is not a self-differentiating 

 structure nor does it respond directly to the 

 thyroid hormone but, rather, it is induced to 

 form the skin window in response to some 

 local factor. 



The identification of any local factors con- 

 cerned proved quite difficult. Helff was able 

 to show that it was not the forelimbs, as their 

 early removal did not prevent the histolysis 

 in the region of the skin window from taking 

 place. By pressure the legs accelerate the 

 actual breaking through of the skin window 

 but this is secondary to the histolysis. De- 

 generating gill tissue, when transplanted to 

 the back, induced histolysis in the overlying 

 skin. Helff concluded that the autolyzing gill 

 tissiie was the inducing agent responsible for 

 the formation of the skin window. This in- 

 terpretation was supported by Van der Jagt 

 ('29), who found that gill tissue increased 

 in potency for the induction of autolysis in 

 the overlying skin as metamorphosis pro- 

 gressed. 



However, this conclusion of Helff is not 

 satisfactory for several reasons. In normal 

 metamorphosis the opercvdar histolysis be- 

 comes evident before (in Rana catesbeiana 

 as much as a month before) any perceptible 

 gill reduction occiu-s. Nor does the gill tissue 

 make the close contact with the area of his- 

 tolysis that Helff's experiment indicates to be 

 necessary for its action to be effective. Fur- 

 thermore, the capacity for inducing histolysis 

 in overlying integument seems to be wide- 

 spread in degenerating tissue, muscle, for ex- 

 ample, also showing this effect. 



Later investigations by European workers 

 showed that, at least in the species investi- 

 gated, different areas of skin have different 

 potencies for skin window development. 

 Opercular skin transplanted to the tail under- 

 went histolysis when the tail resorbed, 

 whereas skin from the back did not. In one 

 species (i?. ridibunda) opercular skin formed 

 a perforation when transplanted to the back 



where no degenerating muscle underlay it 

 (Blacher, Liosner and Woronzowa, '34; Lios- 

 ner and Woronzowa, '35). In a reinvestiga- 

 tion of this problem Helff ('39) likewise 

 found that the skin area was not entirely 

 without determination. He concluded that 

 induction by degenerating gill tissue, by de- 

 generating skin glands and by self-differen- 

 tiation (i.e., direct response to hormone) 

 each plays some part in the formation of the 

 skin window. However, it would seem that 

 until the tissue whose histolysis is active in 

 inducing skin window formation is definitely 

 identified (as explained above it does not 

 seem possible that it is gill tissue) the exact 

 role of self-differentiation cannot be deter- 

 mined because the experimenter cannot be 

 sure that the skin has not previously been 

 exposed to some inductive influence when it 

 is taken for transplantation. Recent work by 

 Kaltenbach ('49) has shown that a fairly 

 complete response can be elicited by implan- 

 tation of thyroxine pellets to the neighbor- 

 hood of the prospective skin window c'rea, 

 thereby greatly strengthening the case for 

 self-differentiation. 



Other suggestions of dependent differentia- 

 tion of tadpole tissues have been made from 

 time to time, for example, that cutting off 

 of circulation to the tail by the growth of the 

 urostyle leads to tail resorption. This crude 

 concept was disproved by Helff ('30), who 

 showed that the tail resorbed normally after 

 extirpation of the urostylar primordium. 



Insects. That the hormonal factors in in- 

 sect metamorphosis operate in many cases 

 directly on the tissues seems evident. For 

 example, implanted bits of skin lying in 

 the haemocoel or in the fat body molt 

 (Piepho, '38; Bodenstein, '44) and imaginal 

 discs continue their development (Boden- 

 stein ,'43 ). Most striking is the fact reported 

 by Williams ('47) that isolated abdomens 

 which have been largely cleaned out of in- 

 ternal organs except the heart, still are able 

 to respond to prothoracic gland stimula- 

 tion. 



Although the few insect tissues that have 

 been studied appear to respond directly to the 

 metamorphic stimulus, indirect action can- 

 not be excluded. It must be realized that in 

 amphibians it is with regard to intra-organ 

 details that Helff found indirect action of 

 the metamorphic stimuli. Such intra-organ 

 details have not yet been explored in insects 

 with respect to metamorphic changes. It is 

 worth noting, however, that local effects of 

 one part upon another art well recognized 

 in the sex organs in this group (Stern, '41). 



