r; 



496 



phagocytic histolysis consists always of a removal of dead 

 tissues by leucocytes or their absorption by embryonic cells; 

 if these cells do not intervene, the dead tissues will dissolve 

 of their own account in the blood. All attempts to explain 

 metamorphosis, therefore, which have concerned themselves 

 with the phagocytosis of living tissues, have proved unsuc- 

 / cessful. Metchnikofif, for example, in seeking to explain the 

 immunity of the imaginal cells at a time when the larval cells, 

 which had but a day before been in the height of their 

 activity, were becoming overwhelmed by leucocytes, concluded 

 that the imaginal cells must emit substances which held the 

 leucocytes at bay, and that the larval cells at metamorphosis 

 no longer did this. He was led to this conclusion by his 

 belief in the existence of anti-leucocytic substances in virulent 

 anthrax bacteria which were not phagocytised. 



Of the death of the cells before phagocytosis there can, 

 however, be no doubt. While this view has more than once 

 been favourably accepted, the cause of the extensive cell-death 

 has not, so far as I am aware, been satisfactorily explained, 

 and this, after all, is the real mechanical principle that under- 

 lies metamorphosis. It is usually believed that the extensive 

 cell -death has been produced by the "wearing out" of a great 

 number of cells all at one time. Such wearing out of cells is 

 believed to occur also in other organisms, but here it is 

 gradual, and as some cells die others replace them. The latter 

 must then be the imaginal cells of the metabolic insect, and 

 metamorphosis has been evolved by the dying cells all * 'learn- 

 ing" to undergo senescence at one moment; this has evidently 

 been evolved in response to the necessity for a metamorphosis 

 in animals whose young and adult stages have different feed- 

 ing habits (Lubbock). 



Now this explanation is clearly not very satisfactory; if 

 the extensive cell-death is merely a concentration, at one 

 moment, of the deaths of numerous cells which would normally 

 take place gradually throughout the life of the insect, how 

 are we to explain rejuvenation of cells in metamorphoses of a 

 simple type? Metamorphosis by cellular rejuvenation would 

 rather appear to be a stage intermediate between a simple 

 development and a total disruption followed by redevelopment 

 from imaginal cells, as seen in the more profound meta- 

 morphoses. Moreover, the essential thing to show would be 

 "how this concentration of death points has been produced, 

 and this is manifestly beyond the scope of modern cytology. 



The extensive tissue disruption, it seems to me, is to be 

 explained on a much simpler principle. The larva consists, 

 in the highly specialized forms, of very specialized cells. Such 

 cells never proliferate, and the growth of the larva is due 



