FATE OF THE LEUCOCYTES 685 



The fat-body. - - The larval fat-body is also destroyed through the 

 activity of the leucocytes in the same way as the other tissues. 

 The reformation of the fat-body seems to begin in the mesoderm 

 of the imaginal buds. Possibly, also, the masses or collections of 

 embryonic cells which are regarded by Schaeffer as "blood-form- 

 ing cells," may serve to regenerate the fat-body. At all events, 

 they have been derived from the mesodennal tissues. Though 

 Wielowiejsky saw the fat-body of Corethra arising from a cell-layer 

 situated under the hypodermis, yet it is not necessary to regard 

 this observation as favorable to the view of Schaeffer that in Musca 

 the larval fat-body is derived in part from the hypodermis, and in 

 part from the tracheal matrix, thus from the ectodermal tissues. 

 (Korschelt and Heider.) 



Definitive fate of the leucocytes. We have seen that the formation 

 of the organs of the imago originates in the imaginal buds, in all 

 cases where these do not pass directty from the larva into the pupa. 

 The leucocytes, whose numbers in the pupa are greatly increased, 

 take no direct part in the formation of the tissues. Their impor- 

 tance seems to lie in this, that they destroy those larval organs 

 doomed to destruction, the parts of which they take in and digest, 

 and possibly, by their powers of locomotion, convey particles of food 

 to the developing organs. 



What, on the other hand, is the fate of the leucocytes after the 

 developmental processes in the pupa have ceased ? There can be no 

 doubt that a part of the so-called granule-cells are again transformed 

 into normal blood-corpuscles. Another, and, as it seems, more con- 

 siderable, share suffer degeneration. Finally, the leucocytes them- 

 selves serve as nourishment for the newly formed tissues. Of 

 interest in this direction is the observation of Van Rees, that 

 numerous leucocytes finally pass into the newly-formed hypodermis 

 and then degenerate in crevices between the hypodermis-cells. 

 (Korschelt and Heider.) 



It has been suggested by Van Rees that the phagocytes attack all the larval 

 organs indiscriminately, but that the active metabolism of the imaginal buds 

 preserves them from these attacks. He also thinks that Kowalevsky is probably 

 right in supposing that the buds render themselves immune by some poisonous 

 secretion. 



Pratt, however, thinks that the supposition of a protecting or poisonous 

 secretion is scarcely necessary to account for the phenomenon, and suggests that 

 the larval tissues arc a prey to the phagocytes, because at the end of larval life 

 they become functionlcss and inactive, so as to become an easy prey to phago- 

 cytes or disintegrating influences of any sort. On the other hand, the imaginal 

 buds "in which there is an exceedingly active metabolism, and all the larval 



