INCEPTION OF INSECT METAMORPHOSIS. 331 



through the nerves, but that it has rather the function of an organ 

 of internal secretion, in that it affects the organism by means of a 

 substance (or substances) which may be supposed to pass into the 

 blood of the caterpillar from the brain at a certain stage of the 

 larval life. Experiments bearing on the transplantation either of 

 the organized brain or of its matter might perhaps decide whether 

 this conclusion is true. It is most probable that the chemical sub- 

 stance (or substances) here acting is the ferment called " thyrosi- 

 nase," the aggregation of which during the pupation of insects and 

 the disappearance of which at the end of the pupal life has been 

 previously noticed by Dewitz ('05, '16) and more recently by 

 Steche and Waentig ('13). In his numerous papers Dewitz long 

 ago ascribed an important " role " to " thyrosinase " in metamor- 

 photic processes. 



The mere observation that the brain has an influence on the 

 inception of histolytical processes in the body of caterpillars gives 

 no idea as to the physiological nature of this influence. The 

 method of removal of the brain from the larval body was not 

 adapted to ascertain whether the proper stimulus effected by the 

 brain is the only factor which excites histolytical processes in the 

 body of the caterpillar, or whether certain physiological changes 

 which appear in the organism cooperate spontaneously and inde- 

 pendently of the brain in the same direction. In other words, does 

 the larval organism undergo metamorphosis exclusively because of 

 the stimulus derived from the brain, or is the brain able to act only 

 when certain physiological changes occur in the tissues of the cater- 

 pillar, changes without which the influence of the brain must re- 

 main powerless? The investigation in this direction had to be 

 made by transplanting the organs of young caterpillars in which 

 these hypothetical physiological changes in the tissues themselves 

 had not yet occurred onto full-grown caterpillars, in which the 

 influence of the brain on the excitement of metamorphosis might 

 already be noticed. Should the grafted organs exhibit acceleration 

 in their development as a consequence of their new surroundings, 

 the metamorphosis of the insects would have to be considered as 

 dependent primarily on the brain, the latter having caused the 

 transformation of the larval organs independently of their age or 

 physiological development. Should this hypothesis prove false, 



