200 INVERTEBRATE PHYSIOLOGY 



discs grow at a rather constant rate ; at least, there seems to be no peak 

 of growth associated with the molting times (Enzmann and Haskins, 

 1938). Growth of these discs seems independent of the cyclic increase of 

 hormone. Indeed, much growth occurs in these organs during the inter- 

 molt periods when the hormone level must be subthreshold. Does this in- 

 dicate that the discs do not depend on the prothoracic-gland hormone for 

 their growth? It does not, for, as mentioned before, these discs trans- 

 planted into an adult male host only grow in the presence of hormone 

 supplied by the ring gland. Growth of these discs is therefore only possible 

 in an environment properly conditioned by prothoracic-gland hormone. 

 The fact that these discs do grow during the larval intermolt period indi- 

 cates the presence of the hormone in the larval system in a titer sufficient 

 for the growth of the discs but inadequate for the induction of molting. 

 The same is apparently true for the imaginal discs of lepidopterous larvae, 

 where the mitotic activity of the epidermis is definitely cyclic but where the 

 discs seem to grow continuously (Eassa, 1953). From all this one must 

 conclude that the ability of the dififerent tissues to respond with growth 

 to a given titer prothoracic-gland hormone varies. Some tissues are able 

 to grow in a low, others only in a higher titer. 



In this connection, one will recall the wound-healing experiments, 

 where the reduced density of the epidermis, brought about by migration 

 of the cells towards the wound, was regulated during the following mitotic 

 cycle. In another insect, Rhodnius, the regulation of the cell density does 

 not await the next mitotic event, but mitotic activity occurs in the de- 

 pleted areas a short time after cell migration (Wiggles worth, 1937). Thus 

 depletion is cause enough to inaugurate cell division. Mitosis in this insect 

 can even be induced without wounding, that is to say without disturbing 

 the normal cell density, just by placing denatured protein material on the 

 outside of the cuticle (Wiggles worth, 1937). Factors emanating from the 

 applied material apparently pass through the cuticle, altering the epidermis 

 cells in such a way as to make them capable of mitosis. These facts do not 

 indicate that Rhodnius epidermis cells can divide in the absence of pro- 

 thoracic-gland hormone. They imply that changes in cell density, as well 

 as the application of certain svibstances, are able to alter the responsive 

 capacity of the cells. The altered cells are now able to respond to the low 

 hormone titer prevailing in the animal during the intermolt period. The 

 epidermal cells of Rhodnius are apparently easier to alter experimentally 

 than those of the roach. In the latter, density changes do not make the 

 cells capable of immediate mitosis. But density changes have altered the 

 roach epidermis cells also ; this is evident from their reaction during the 

 mitotic wave period, where the cells in the regions of low density divide 

 more frequently than those in normal density regions. Of course, the ob- 



