485 



105), and to a less extent in certain other tissues — processes 

 which have all been described above. Nevertheless, if these 

 tissues are inaccessible to leucocytes, as often happens, for 

 example, through the pressure of the fat-body, then phago- 

 cytosis does not occur, and the tissues undergo a gradual 

 solution in the blood (fig. 91, trl.). 



Weismann (1864) was the first to observe tissue disin- 

 tegration in metamorphosing insects. He regarded the tissues 

 as breaking up into minute globules, "Kdrnchenkugeln," and 

 to the process he gave the nam.e histolysis. In 1884 Van Rees, 

 and in the following year Kowalevsky, stimulated by Metchni- 

 koff's great discovery of the phagocytic activity of leucocytes, 

 put a special interpretation upon Weismann^s '' histolysis*' — 

 they regarded it as a tissue phagocytosis, the "Kornchen- 

 kugeln" being the gorged leucocytes. 



Berlese (1901) has wholly denied the existence of phago- 

 cytosis of living tissues; while Perez (1910), working with 

 Calliphora, regards the leucocytes as playing the main part 

 in the destruction of larval tissues. In Nasonia there can be 

 no doubt that chemical disintegration, and phagocytosis of 

 dead tissues, both occur. The phagocytosis of dead tissues 

 is, however, not so ingenious a device for the removal of 

 debris as it at first sight appears to be; a more direct process 

 would obviously be the solution of dead material in the blood. 

 That this can occur in tissues which are phagocytised only 

 when leucocytes have special access to them has been clearly 

 demonstrated in the case of the tracheoles, and phagocytic 

 histolysis is to be looked upon as the sign of a not yet fully,. 

 eyoTved metamorphosis: — of an imperfect though ingenious 

 method for attaining a result— which ...will, he perfected only, 

 wli^Tthe tissues have "learned" to dissolve directly in the 

 blood stream, and the leucocytes, in their turn, to refrain 

 from attacking theae as they disintegrate. Berlese believes 

 he~has ohserved this kind of metamorphosis in a number of 

 insects, but there is little doubt that he overlooked a quite 

 extensive phagocytosis of larval tissues. It is only when other 

 metamorphoses, especially those of highly specialized insects, 

 are investigated, that an "ideal" transformation may be dis- 

 covered. Phagocytic histolysis of larval tissues, indeed, seems 

 to be a very much over-estimated factor in the mechanism 

 of the insect metamorphosis, and although some investigators, 

 e.g., Verson, regard it only as a removal of dead larval 

 tissues, others believe the leucocytes to be endowed with far 

 higher powers and that they destroy the larval tissues while 

 these are yet capable of actively functioning; Metchnikoff 

 inclined to this view, and more recently Perez (1910) has 

 written in its favour. I shall discuss it more fully in the 



