Insects 



341 



pattern a succession of determination streams 

 passes over the wing surface determining 

 the various component parts of this system. 

 The nature of the determining influences 

 is not known. The experimental analysis of 

 wing pattern formation has fmrnished a 

 wealth of information concerning the state 

 of determination of the various zones at dif- 

 ferent times in development and has brought 

 to light the interdependency of these zones 

 in the formation of the entire pattern (Cas- 

 pari, '41). 



DEVELOPMENTAL INTERACTIONS IN 

 LATER STAGES 



Although the different elements comprising 

 the component parts of an organ can still 

 replace each other in organ anlagen of 

 postembryonic stages, the imaginal discs or 

 the prospective regions from which they 

 arise are at these late stages usually com- 

 mitted to the type of organ or body part 

 they eventually form. There are, however, 

 experiments indicating that the plasticity of 

 the formative disc material goes beyond the 

 limit of regulations of its own parts. It has 

 been shown that the developmental path of 

 one type of disc can be changed into an- 

 other. In late postembryonic stages of de- 

 velopment, therefore, the prospective signifi- 

 cance of these discs is not yet organ specific. 

 For example, the presumptive antenna mate- 

 rial of last instar Drosophila larvae can be 

 changed into leg material by appropriate 

 temperatui'e treatment (Vogt, '46b) or x-ray 

 treatment (Waddington, '42). Also in 

 Drosophila the change of presumptive head 

 chitin into eye facets (Steinberg, '41; 

 Chevais, '44) and wing tissue into body skin 

 (Waddington, '42) by similar methods are 

 other instances of the same sort. Develop- 

 mental modifications produced by treating 

 Drosophila larvae or pupae with ether 

 (Gloor, '46, '47) reveal effects with like 

 implications, and so do many other inves- 

 tigations dealing with phenocopies. It is 

 of interest to note further that transforma- 

 tions of presumptive antenna material into 

 leg material were obtained in Drosophila by 

 the use of two chemicals, colchicin (Vogt, 

 '47b) and a nitrogen mustard compound 

 (Bodenstein, '47) — chemicals that are known 

 to have a marked effect on cell division. The 

 long-suspected importance of growth rela- 

 tions (Goldschmidt, '38) between the various 

 organ-fields as decisive factors in determin- 

 ing the type of organ differentiation has 

 thus gained support. The phenomenon 



known as homoeosis, i.e., the replacement 

 by regeneration of one organ by one belong- 

 ing to another region of the body, needs 

 to be mentioned in this connection. In 

 studying this type of regeneration, it was 

 found that regional specific regeneration 

 occurs if relatively little of the original or- 

 gan is removed, while amputation of larger 

 parts is usually followed by heteromorphic 

 regeneration (Brecher, '24). It thus seems 

 that in the first instance the organ-field of 

 the original organ takes the lead in determin- 

 ing the new structure, while in the second 

 instance this lead is taken by a neighboring 

 field. In ontogenesis as in regeneration, there- 

 fore, it is the interaction between zones 

 within one organ-field, or that between 

 neighboring fields, which determines the 

 type of organ finally to be formed. 



An interesting and special case showing a 

 change from one type of development into 

 another has been reported in the moth 

 Orgyia (Lep.). Here, the secondary sexual 

 characters of the female wing discs are still 

 not irrevocably determined in fourth instar 

 caterpillars, for fourth instar wing discs 

 when transplanted into male hosts and made 

 to regenerate in their new surroundings will 

 form male wings. In the reverse experiments, 

 however, transplanted male wings will re- 

 generate male wings in the female environ- 

 ment (Paul, '37). It might well be pointed 

 out that this case is a singular instance of 

 this kind, although the problem has been 

 studied extensively in other insects. The 

 experiments usually quoted as proof for the 

 complete determination of secondary sexual 

 characters are castration experiments and 

 transplantations of gonads between indi- 

 viduals of the opposite sex. It may be added 

 that the above cited case is not only singular 

 in its results but also in its method. In 

 Orgyia, in contrast to all other investigations, 

 the organ exhibiting the secondary sexual 

 character was brought into contact with 

 the entire developing system of the oppo- 

 site sex and hence subjected to influences 

 additional to those of the gonads. It is per- 

 haps the lack of appropriate experimentation 

 rather than the special condition of the 

 Orgyia wing that made the obtained results 

 exceptional. 



Finally, mention should be made of devel- 

 opmental interactions between systems which 

 already possess a considerable degree of dif- 

 ferentiation. There are species of Drosophila 

 that have spiral, and others that have non- 

 spiral, ellipsoid adult testes. Now it has been 

 found (Stern, '41a, '41b) that when vasa 



