208 INVERTEBRATE PHYSIOLOGY 



tially the same result is obtained when corpora allata in varied number are 

 transplanted into last-stage caterpillars, where the allatum hormone titer 

 is normally a low one (Piepho and Heims, 1952). Under the impact of 

 the increased allatum hormone titer, the cuticle responds at the next molt 

 with a varied degree of larval differentiation, instead of with pupation. 

 The more allata are implanted, i.e., the higher the hormone titer is raised, 

 the wider spread is the larval differentiation of the cuticle. A complete 

 larval molt can be induced if the number of allata implanted is large 

 enough. By means of two quite different procedures, namely by the re- 

 duction of the allatum hormone titer in the young organisms or by the 

 increase of this hormone titer in the last-stage caterpillar, it is possible 

 to change the normal sequence of differentiation. 



Changes in Target Organ Differentiation Thresholds 



The ability of epidermal cells after wounding to restore their normal 

 cell density by increased cell division and the induction of division by the 

 application of certain materials to the surface of the skin have been cited 

 to illustrate how special conditions not affecting the humoral situation 

 can alter the responsive capacity of target cells. An interesting phenom- 

 enon with like implication was found in experiments on wound healing 

 in relation to larval differentiation (Piepho, 1950). The experiments are 

 as follows : Several small wounds were made in the cuticle of last-stage 

 wax-moth caterpillars. A few corpora allata were then implanted into one 

 of these wounds. Before these animals molted, the wounds healed by the 

 formation of a thin wound epithelium which was protected on the outside 

 by a cover of coagulated blood. These individuals then molted into pupae 

 which were normal, except that at the wound regions the epidermis, in- 

 stead of laying down pupal cuticle, responded with the formation of larval 

 cuticle. The implication is clear. The threshold of the epidermal cells for 

 larval differentiation has been lowered by regeneration. The epidermis, 

 we may say, has been rejuvenated, for it responded with larval differen- 

 tiation to an allatum hormone titer that was too low to elicit a larval res- 

 ponse in the nonregenerated epidermis. The high sensitivity of the regen- 

 erated epidermis can also be maintained beyond the first postoperative 

 molt, as the following observation demonstrates. Again, a wound was 

 made in the cuticle of the last-stage larva, but this time ventrolaterally in 

 the region of an abdominal proleg. Then six corpora allata were trans- 

 planted. This caterpillar molted into a pupa which showed at the wound 

 region a rather large area made up of larval cuticle. From within this larval 

 region, a complete larval proleg protruded. The pupa finally molted again, 

 giving rise to the imago, where, surrounded by the scaled imaginal skin, 

 the larval area containing the larval proleg was still present. Therefore, 



