INSECT GROWTH AND DIFFERENTIATION 203 



Regeneration and Growth 



Attention must now be given to another aspect of development — namely, 

 regeneration, in which growth plays so important a role. If the prothoracic- 

 gland hormone is as vital for the control of mitotic activity as the fore- 

 going considerations seem to indicate, its effects should certainly be de- 

 monstrable in the processes of regeneration. Recent evidence provides 

 proof for this contention (Bodenstein, 1955). Although most insects re- 

 generate their legs readily in the immature stages, they lose this power in 

 the adult stage. Legs of adult cockroaches amputated at the femur-trochan- 

 ter level are never replaced by regeneration. However, the transplantation 

 of prothoracic glands into such adult individuals restores the power of re- 

 generation. Under the influence of the hormone, the adult animals are 

 caused to molt. When this occurs, the amputated leg is replaced by a well- 

 formed regenerate. Since growth is an integral part of the process of re- 

 generation, the importance of a hormone-conditioned environment for the 

 expression of growth is again evident. 



Now the roach is an insect in which cell multiplication is a cyclic afifair. 

 One may therefore assume that during the intermolt periods there prevails 

 a hormone titer too low for the support of growth. Yet, as far as regenera- 

 tion is concerned, this titer is adequate for the growth of the regenerating 

 tissues, for regeneration starts and continues after amputation of the leg 

 at any time during the nymphal intermolt period. This implies that the re- 

 generating cells respond with growth to a hormone titer which is too low 

 to bring about the same reaction in nonregenerating cells. One will recall 

 that young cells exhibit the same high sensitivity to the hormone. In this 

 respect, the behavior of the regenerating cells resembles that of young 

 cells. Since regeneration recapitulates in many aspects early develop- 

 mental processes, the characteristic growth response of the regenerating 

 cells is not astonishing. As in young cells, the growth rate of the regenerat- 

 ing cells should fall off with age. Since, concomitantly with the decrease 

 in the responsiveness of the regeneration cells, the hormone titer increases 

 as the animal gets closer and closer to the molting stage, the overall growth 

 velocity of the regenerate is the expression of these two interacting forces. 

 From this point of view, regeneration might be looked on as a special case 

 of growth regulation. 



Growth of Adult Target Organs 



The adult insect has no prothoracic gland and is therefore unable to molt 

 and hence to grow. Yet, if the adult is experimentally supplied with pro- 

 thoracic-gland hormone, mitotic activity can be induced and the insect 

 made to molt again. The adult tissue has retained its powers of growth. 



