INSECT GROWTH AND DIFFERENTIATION 199 



effective threshhold ; during the nonmitotic intermolt periods there is 

 apparently too little growth hormone in the system to be effective. The 

 experimental evidence for the above assertions is as follows. 



Cyclic Growth 



The necessity of the prothoracic-gland hormone for growth is well 

 illustrated in experiments on Drosophila (Bodenstein, 1943). Imaginal 

 discs transplanted into the abdominal cavity of adult male f^ies do not 

 grow. But growth in these organs can be induced by the simultaneous 

 transplantation of ring glands, which in these animals produce the pro- 

 thoracic-gland hormone. Thus only in the presence of the hormone is 

 mitotic activity possible. Similarly, in tissue culture experiments ( Schmidt 

 and Williams, 1953) one finds that spermatogonia of Platysamia cecropia, 

 isolated in a hanging drop of caterpillar blood, divide only when blood 

 containing prothoracic-gland hormone is used ; no growth occurs in sper- 

 matogonia isolated in blood containing no prothoracic-gland hormone. 

 The humoral dependence of the cyclic mitotic wave in the cells of the epi- 

 dermis is also well documented. Mitosis in the epidermis takes place only 

 when prothoracic-gland hormone titer has reached a certain threshold 

 shortly before molting (Wigglesworth, 1934; Kiihn and Piepho, 1938). 

 Mitosis never occurs when the hormone concentration is prevented experi- 

 mentally from reaching the effective level. In the other hand, whenever a 

 molt is induced experimentally, mitotic activity in the epidermis is also 

 induced. No mitosis is seen in the epidermis during the intermolt period, 

 when the prothoracic-gland hormone titer is expected to be very low. 



The cyclic rise of the hormone level and the associated induction of 

 mitosis are also evident in wound healing. If small wounds (needle pricks) 

 are made in the ventral coxal skin of the cockroach leg, they are closed by 

 a migration of the epidermis cells over the wound surface. This depletes 

 the area surrounding the wound of cells ; thus it lacks a normal cell density. 

 This situation is regulated during the next molt when, under the influence 

 of the normal prothoracic-gland hormone level, the mitotic wave starts in 

 anticipation of this molt. At this time, increased cell division at the depleted 

 areas brings back the normal cell density (Bodenstein, unpublished). In 

 similar experiments on the cuticle of P. cecropia, much the same observa- 

 tions were made, for here too it is at the time of the occurrence of the 

 mitotic wave and not before that the deficient cell number is restored 

 (Smith and Schneiderman, 1954). 



Continuous Grozvth 



Quite different is the course of events in the growth behavior of the 

 imaginal discs of Drosophila larvae. During the entire larval period these 



