INSECT GROWTH AND DIFFERENTIATION 209 



the regenerated epidermis had maintained its high sensitivity to the al- 

 latum hormone through the period of two successive molts. This case 

 strikingly illustrates the importance of the target's response in determin- 

 ing the final developmental achievement. 



As indicated above, regeneration seems to rejuvenate last-stage epi- 

 dermis, making the wound epidermis more responsive to allatum hormone. 

 The validity of this contention gains support by one further ingenious 

 observation and experiment by Piepho and Heims (1952). These inves- 

 tigators found that the skin of wax-moth caterpillars is composed of two 

 types of cuticle. The skin of the larval head, pronotum, and anal plates is 

 made up of a brown, hard cuticle. The skin of the rest of the body consists 

 of a soft yellowish cuticle. Now the hard, brown cuticle, of which the entire 

 pupa is made, very much resembles the larval head, pronotum, and anal 

 plate cuticle in its morphological as well as its histological structure. The 

 cuticular pattern of the caterpillar is a mixture of larval and pupal cuticle. 

 Thus, in normal development, the larval epidermis reacts to a normal lar- 

 val allatum hormone titer with the formation of two cuticular types — a 

 larval and a pupal type. One will recall that the pupal type of cuticle is 

 much less sensitive to the allatum hormone, but that by regeneration it 

 can be sensitized. If the hard, dark larval cuticle is actually the physio- 

 logical equivalent of the pupal cuticle, it too should be sensitized by re- 

 generation. To test this, a small window was cut in the hard, dark cuticle 

 of the pronotum and the pre-anal plate of young third-stage caterpillars. 

 It was observed during the following molts that the w^ound epidermis in 

 some of the cases actually formed soft, yellowish cuticle, which stood out 

 in striking contrast to the surrounding dark cuticle. The epidermis of 

 the larval pronotum and pre-anal plate can indeed be sensitized by regen- 

 eration. Without the addition of extra allatum hormone, but under the 

 influence of a normal larval allatum hormone titer, the dark larval cuticle 

 can be made to form cuticle of the soft yellowish type. It is possible, the 

 authors believe, that the other larval sclerites also possess a slightly lower 

 sensitivity to the allatum hormone than the nonsclerotized region of the 

 body. The typical larval cuticular pattern is thus basically a sensitivity 

 pattern. In normal development, the epidermis reacts according to its in- 

 herent sensitivity pattern to the prevailing allatum hormone titer, which 

 results in the differentiation of a cuticular pattern, typical for each stage. 



Regeneration and Differentiation 



The knowledge gained from experiments on wound healing has been 

 supplemented by studies on regeneration. The role of hormones in the 

 growth phase of regeneration has been discussed. As far as the differen- 

 tiation of the regenerate is concerned, the evidence suggests that it too 



