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larval tissues underwent a process of disintegration — 

 '"histolysis" lie called it — into rounded bodies, wliich he called 

 Kornclienkugeln, and that the imago in l^^urn was formed 

 from small areas of cells, which Swammerdam had already 

 discovered, though he had not recognized their significance; 

 to these he gave the name "imaginal discs." He was able 

 to demonstrate the sexual organs in a young condition in 

 the larva, and to show that the insect metamorphosis was 

 entirely different from the alternation of generations that 

 occurred in some groups of animals and plants. He demon- 

 strated the occurrence of metamorphosis in most of the organs 

 of the body, including the heart and nervous system, which 

 other investigators with, more elaborate technique at their 

 disposal have since questioned; and though his observations 

 were necessarily incomplete, and did not extend largely to 

 cell changes, yet his conclusions were, in the main, correct. 



Since Weismann's memoir the blow-fly (GalUfhora) has 

 been used by a number of investigators for the study of 

 metamorphosis, so that our knowledge of the process in this 

 insect, though still very incomplete, is much fuller than that 

 of any other. In 1876 the. Russian Ganin wrote upon it, 

 and described the imaginal ''nests" within the intestine. 

 In 1884 Van Rees, and in the following year, quite inde- 

 pendently, Kowalevsky, guided by Metchnikoff's recent dis- 

 covery of the phagocytic action of leucocytes, showed that the 

 larval tissues were destroyed by the interference of these 

 colourless corpuscles of the blood. A special interpretation 

 was therefore placed on Weismann's histolysis, and the 

 '"Kornchenkugeln" .proved to be nothing but gorged phago- 

 cvtes, a fact the truth of which Metchnikoff had himself 

 already perceived from the drawings given by Ganin. 



Since that time a number of other observers have added 

 details to the knowledge accumulated by the earlier workers : 

 — Van Rees studied it in 1888; Lowne published a few 

 observations (mostly incorrect) in 1890-1895 ; Vaney wrote 

 about it in 1902 ; while Perez published his very detailed 

 work in 1910. 



In 1899, and later in 1901, Berlese published his observa- 

 tions, and seriously questioned the important role which the 

 leucocytes were believed to play in the removal of the larval 

 tissues. From the earlier writings it seemed to follow that 

 the leucocytes attack the living tissues, so that metamorphosis 

 is, in part, brought about by more than usually highly- 

 endowed leucocytes. Berlese denied this conception entirely. 

 As he appears to have been misunderstood by others, it is 

 best to quote his own words (1901): — "Phagocytosis never 



