324 



occurs, and amoebocyfces only become active when the muscle 

 has disintegrated through internal causes." By phagocytosis 

 he evidently means the phagocytosis of living tissues; and 

 his "amoebocytes" appear to be a, congregation of various 

 kinds of embryonic cells and leucocytes, though he does not 

 specially mention these. Perez (1910), on the other hand, 

 has taken precisely the opposite view, and regards the 

 leucocytes as playing the main part in the destruction of 

 tissues. "I think I have proved satisfactorily that the dis- 

 integration of the muscle is due to phagocytes, and that there 

 is no spontaneous fragmentation of this organ into sacrolytes, 

 as Berlese thought." I may say at once, that the study of 

 the metamorphosis of Nasonia has led me to conclude that 

 while neither statement is quite correct neither is wholly 

 / r wrong; phagocytes play a large part in the removal of larval 

 L tissues, but such tissues are always dead. 



Besides the observations of these workers, others have 

 been made on portions of the metamorphosis of other insects, 

 but nothing sO' extensive as those made on the blow-fly exists. 

 In 1875-1878 Kiinckel d'Herculais published his studies on 

 the structure and transformation of the syrphid fly Volucella; 

 Deegener in more recent years has studied the transformation 

 of the intestine in a number of insects; and Verson (1898) 

 examined it in the silkworm. Perez (1902) examined por- 

 tions of the metamorphosis of the ant Formica rufa; Bauer 

 studied the transformation of the brain in several insects; 

 and in 1912 Giinther investigated the development of the eye 

 in Dytiscus. In 1910 Poyarkoff published his very interest- 

 ing observations on the metamorphosis of a beetle, Galeruca; 

 he showed that, while some organs underwent the usual type 

 of phagocytic histolysis, others (the integument and part of 

 the intestine) passed through a remarkable process of cellular 

 rejuvenation. 



It may be said then, that while we possess a considerable 

 knowledge of the main features of insect metamorphosis, on 

 some of the fundamental facts much difference of opinion 

 prevails. Why do the larval tissues disappear ? Do the 

 phagocytes kill them, or do they merely remove them after 

 they have died ? If the latter, then how is their death brought 

 about? If in one insect phagocytic histolysis occurs, and in 

 another merely cellular rejuvenation, how are we to correlate 

 the processes ? It is these questions that I shall attempt to 

 answer in the present paper. The histological changes under- 

 gone by some of the larval organs, moreover, have never been 

 examined — heart, peripheral nerves, ventral nerve cord, and 

 others ; whilst the greatest differences of opinion prevail about 

 the details of other organs such as the muscles and intestine. 



