Metamorphosis 



647 



for the analysis of morphogenic factors. 

 Work on individual tissues or enzyme sys- 

 tems holds much greater promise. Enzyme 

 analysis has been most successfully applied 

 to insects. Williams ('51) and his collabora- 

 tors, who have studied the cytochrome sys- 

 tem, find a close correspondence between 

 cytochrome c and the activity of prothoracic 

 gland hormone. The emergence from dia- 

 pause is correlated with a shift from a cy- 

 anide-insensitive flavoprotein oxidation sys- 

 tem to a cytochrome system sensitive to cy- 

 anide. By inactivating cytochrome oxidase 

 with carbon monoxide and other compounds 

 Williams was able to stop the post-diapause 

 growth processes in the pupa. The cyto- 

 chrome system in Drosophila (Bodenstein 

 and Sacktor, '52) also parallels the growth 

 activity, although perhaps not the hormone 

 level. Although only preliminary results are 

 available at present it would appear that en- 

 zymatic analysis holds great promise of an 

 understanding of hormone activity in mor- 

 phogenesis. This approach should be ex- 

 tended to the amphibians, where the varia- 

 tions among the tissues in responsiveness to 

 thyroid would seem to provide favorable 

 material for such analysis. 



DIRECT AND INDIRECT TISSUE 

 RESPONSE 



Amphibians. The demonstration of a hor- 

 monal stimulant in the blood raises the 

 question as to whether the responses of the 

 individual tissues are all made directly to 

 this stimulus or whether some at least are 

 not indirect responses induced by neighbor- 

 ing tissues. This question is most conven- 

 iently explored by transplanting tissues be- 

 tween animals metamorphosing at different 

 times. If the graft undergoes metamorphosis 

 synchronously with the host tissue, then it 

 may be concluded that it is responding di- 

 rectly to the metamorphic stimulant rather 

 than indirectly through some neighboring 

 organ or by self-differentiation. Such evi- 

 dence of direct response in amphibians exists 

 for gills (Kornfeld, '14), skin (Uhlenhuth, 

 '17; Helff, '31a), intestine (Sembrat, '24), 

 tail muscle (Helff and Clausen, '29; Fukai, 

 '34), tongue (Helff, '29), and eye (Vrtelowna, 

 '25; Schwind, '33). In some cases the tissue 

 response is one of growth, in others one of 

 histolysis or a combination of the two proc- 

 esses. 



A particularly striking type of evidence 

 of direct tissue response was secured by Koll- 

 ros and collaborators (Kollros, '42; Kalten- 



bach, '49). For example, Kollros implanted 

 bits of thyroid directly into the tadpole hind- 

 brain and thereby brought about a localized 

 maturation of the lid-closure reflex mecha- 

 nisms. By irradiating parts of the tadpole with 

 x-rays Puckett ('37) was able to inhibit the 

 growth-promoting effects of thyroid and 

 thereby separate the growth from the resorp- 

 tion actions of the thyroid hormone, showing 

 that these responses are independent of each 

 other. 



On the other hand, Helff and his associates 

 have been able to show that some tissue 

 changes are not direct responses to hormones 

 but are dependent vipon stimulation from 

 neighboring structures. The formation of the 

 tympanum in the frog occurs during late 

 stages in the metamorphic climax. The pro- 

 duction of this structure involves a modifi- 

 cation of the integument and also the forma- 

 tion of a connective tissue sheet, the lamina 

 propria, containing peculiar yellow fibers. 

 Helff ('28) showed that skin from any region 

 transplanted to the ear region was induced 

 to form tympanum. Conversely skin from the 

 ear region transplanted elsewhere failed to 

 form tympanvim. By transplanting the an- 

 nular tympanic cartilage he was able to 

 show that this cartilage was the source of 

 an inductive influence leading to the forma- 

 tion of tympanic membrane in skin overly- 

 ing it; this inductive effect was present, 

 though weaker, in killed cartilage (Helff, 

 '40). Helff ('34) also found some capacity 

 for tympanum induction in the quadrate 

 and suprascapular cartilages which, because 

 of their positions, can have no influence in 

 the formation of the tympanum in normal 

 metamorphosis. The inductive capacity of 

 the annular cartilage, moreover, persisted for 

 some time after the completion of the mem- 

 brane in normal metamorphosis. However, 

 the yellow fibers of the lamina propria were 

 shown to form only under influence of con- 

 tact with the columella (Helff, '31c). Pre- 

 sumably the tympanic and columella carti- 

 lages develop under direct thyroid stimula- 

 tion since on transplantation each continues 

 at least partial development (this point was 

 not directly explored by Helff). 



A more complex case in which inductive 

 phenomena play a part is that of the forma- 

 tion of the skin window or opercular per- 

 foration, for the foreleg in Anura. This is a 

 definitely circumscribed area of skin of the 

 operculum which undergoes autolysis in the 

 late prometamorphic period (Fig. 217). The 

 forelegs which have meanwhile developed in 

 the opercular cavity are thrust through the 



