495 



however, take the opportunity, and nourishing themselves 

 upon the highly nutrient material of the dead larva, grow by an 

 orderly process of development into a totally different organism 

 — ^the imago. Even the instincts of the metamorphosing 

 larvae are those of dying animals — ^they avoid the light; they 

 roll themselves up in leaves; they crawl into secluded spots, 

 or, significantly, even bury themselves in the earth ! And yet 

 by a wonderful process of development, unique of its kind 

 among living things, the dead larva becomes the prey of 

 minute sjcattered cells which have lain helpless among the 

 larval cells while these fl.ourished; having awaited their oppor- 

 tunity, these now spring into activity, and nourisliing them- 

 selves upon the bodies of the dead larval cells, form another 

 organism. From the grave of the dead larva arises the perfect 

 insect. 



While then in the simpler metamorphoses there is a rapid 

 transformation of tissue, in the more profound type a serious 

 disruption of tissues occurs, the embryo developing within 

 the old larval sheath is in a helpless condition, and we speak 

 of it as the pupa. ^ ^^ ' 



The metamorphosis of an insect consists, therefore, of two 

 processes — a process of disruption and a process of orderly 

 embryonic development. A "complete" explanation of meta- 

 morphosis will, therefore, have to account for the disruption 

 and likewise for the orderly development which ensues. The 

 mechanism of the development is identical with that of any 

 other embryonic development; it is the unexplained *'Ent- 

 wicklungsmechanik der Organismen," and I can say nothing 

 of it here. For the process of disruption — of actual trans- 

 formation — however, a simple explanation may, I think, be 

 given. As it is highly probable, moreover, that the factor 

 which brings about disruption of larval tissues in the more 

 complex metamorphoses is identical with that which forces 

 the cells in a simpler type to rejuvenate, it should be possible 

 to obtain a general principle underljdng metamorphosis. And 

 lastly, since the rejuvenation of individual cells is itself evi- 

 dently a process of rapid differentiation in cells in which the 

 process has for some reason become temporarily suspended, 

 it would seem that the same principle should be responsible for 

 the cell differentiation seen in "normal" embryonic develop- 

 ments. The discovery of the mechanism of metamorphosis 

 ought, therefore, to lead to a more general theory of cell 

 differentiation , 



As the principle is most clearly revealed in the more pro- 

 found metamorphoses such as that of Nasonia, or the Muscids, 

 these will be considered first. In the section on cell degenera- 

 tion in the larval tissues of Nasonia I have pointed out that 



