Insects 



343 



nature of the mechanism of determination 

 in discs in general. 



Now it should be understood that hormones 

 are not merely general stimulators, creating 

 a situation through which the invisibly 

 fixed organization of the organic pattern 

 becomes realized. They do more, for they 

 often decide specifically which of the de- 

 velopmental possibilities existing in the 

 reacting material shall become manifest. In 

 fact, it is often only through the specific 

 action of these hormones that we can detect 

 the existence of latent developmental poten- 

 tialities. Take the case of the young cater- 

 pillar epidermis. Under normal conditions, 

 the young epidermis at the ensuing moult 

 would have laid down caterpillar cuticle 

 under the influence of the juvenile hormone. 

 Yet, under experimental conditions, pre- 

 maturely supplied with metamorphosis hor- 

 mone, the same epidermis lays down pupal 

 cuticle. In the reacting material, therefore, 

 several competences must have existed side 

 by side and the hormone decided when and 

 which of these became manifest. As far as 

 the determination of definite fates is con- 

 cerned, the young larval epidermis is but 

 labilely determined, for by the appropriate 

 hormone it can be switched from larval to 

 imaginal development. Not only epidermis 

 but also organ anlagen show this dual type 

 of determination (Bodenstein, '42). Appar- 

 ently a similar state of affairs prevails in the 

 reversal of secondary sex characters in the 

 Lepidoptera wing cited above. Here influences 

 presumably hormonal in nature emanating 

 from the organic environment were able to 

 shift the development from a female into a 

 male direction. Even at the end of develop- 

 ment, when the adult organism emerges, it 

 seems that absolute stability, i.e., a definite 

 determination of the parts to pursue fixed 

 fates, is never reached. This is most strik- 

 ingly demonstrated by the fact that adult 

 insects can be made to moult in the presence 

 of juvenile hormone and that part of the 

 organism now possesses nymphal or larval 

 character (Wigglesworth, '40; Piepho, '38a). 

 The developmental systems necessary for 

 this reversion of development were hence 

 latent in the cells, but needed special condi- 

 tions for their realization. There are other 

 instances with like implication. One of special 

 interest is the case of the walking-stick 

 Dixippus in which, by extirpation of the 

 corpora allata, the normal hormonal balance 

 was upset; this resulted in the formation of 

 an eye-like structure in the ectoderm of the 

 head. This eye, though never normally ob- 



served in this species, is characteristic for 

 related species (Pflugfelder, '39a). Now the 

 purpose of presenting this information is to 

 emphasize the fact that apparently rigidly 

 determined systems may still contain latent 

 developmental powers of amazing plasticity 

 which, however, become manifest only under 

 certain conditions. 



The role of hormones in growth has been 

 mentioned. It is not astonishing that we find 

 them playing an important part in the activa- 

 tion of growth as a component process in 

 regeneration. The walking-stick Dixippus 

 loses its ability to regenerate new append- 

 ages in the adult stage, while it freely re- 

 generates these organs in nymphal stages. 

 If the imago is induced to moult again by the 

 transplantation of young corpora allata and 

 thus supplied with juvenile hormone, the 

 animal once again regains its powers to re- 

 generate. Conversely, extirpation of the cor- 

 pora allata in young nymphs results in the 

 loss of the regeneration power, while reim- 

 plantation of these glands restores this capa- 

 city (Pflugfelder, '39a, '39b). Removal of 

 the corpora allata in young nymphal stages 

 also causes severe changes in all tissues 

 (Pflugfelder, '38; Scharrer, '46). Many tis- 

 sues degenerate, others show abnormal un- 

 controlled growth often resembling certain 

 tumors familiar to vertebrate pathologists. 

 Mesodermal organs seem to be affected first, 

 and more drastically than skin. Normal con- 

 ditions may be restored by providing the 

 abnormal animals again with corpora allata. 

 Internal secretion and tissue proliferation as 

 well as cell pathology are closely related. In 

 general, we can visualize the part played by 

 hormones in insect development as follows: 

 In addition to their more specific actions in 

 controlling the manifestation of certain 

 morphogenetic and histogenetic features in 

 development, the hormones play the more 

 general role of metabolic regulators. By 

 virtue of their reintegrating function, they 

 bring the various parts of the organism 

 under a common control. Thus any dis- 

 turbance of this prominent integrating sys- 

 tem results in severe developmental modifi- 

 cations and functional alterations. 



REFERENCES 



Auerbach, C. 1936 The development of the legs 

 and the wings and halteres in wild type and some 

 mutant strains of Drosophila melanogaster. Trans. 

 Royal Soc. Edinburgh, 53.-787-815. 

 Bock. E. 1939 Bildung und Differenzierung der 

 Keimblatter bei Chrysopa perla (L). Z. Morph. 

 u. Okol. Tiere, 35;6l5-700. 



