PSYCHE. 



proper, the real anlagc of the extremitv, 

 while the portion at the side becomes 

 the parapodial membrane, and the disc 

 is formed as we find it throughout tlie 

 larval period (Fig. 3). 



As to the discs of the internal organs 

 and of the abdominal hypodermis, I 

 have not observed them in the embryo 

 and think it is probable tiiey appear tirst 

 in the larva. 



Before closing I wish to dwell for a 

 moment on one or two theoretical ques- 

 tions which naturally present them- 

 selves. In the first place, why is it that 

 in the brachyceran dipters the phago- 

 cytes, that is the blood corpuscles, 

 during the metamorphosis do not attack 

 all the larval tissues indescriminatelv 

 instead of being selective in their oper- 

 ations. We find that in the histolysis 

 certain organs only, such as the larval 

 hypodermis, intestine, muscles, etc., are 

 attacked and destroyeil while others, 

 such as the imaginal discs, the heart, 

 central nervous system, reproductive 

 glands, and even some of the muscles are 

 left intact. Kowalevsks* seeks to an- 

 swer this question. He says the reason is 

 the same as that which accounts for tiie 

 fact, first observed by Metschnikoft'.t 

 that certain virulent bacteria, as the 

 form which is the cause of Anthrax, are 

 not attacTked by leucocytes, while the 

 same form in Pasteur's vaccine for An- 

 thrax, which has been weakened ant! 

 robbed to a certain extent of its \-iruIent 

 power, is attacked and consumed. With 



*1. c. p. 555. 



t Metschnikoff. Arcliiv f. pathol. An.it. Bd. c,?, p. 51^. 



the beginning of the pupal stage in 

 Musca the larval tissues, being no longer 

 active, functional tissues, become weak 

 and degenerate and are attacked and de- 

 stroyed bv the amoeboid blood cor- 

 pu.sclesof the pupa, as are the non-viru- 

 lent bacteria of the vaccine by the leu- 

 cocytes. The imaginal discs, on the 

 other hand, which are a fresh, voimg 

 tissue, whose cells are rapidly proliferat- 

 ing, are immune, as are also the viru- 

 lent bacteria. Kowalevsky also savs 

 that perhaps the tissue of the imaginal 

 discs also secretes some sulistnnce which 

 renders it secure against the attacks of 

 phagocytes as do the virulent Bacteria. 

 Van Reesf says that the phagocytes do, 

 .IS a matter of fact, attack all the larval 

 organs indiscriminately, but that the 

 active metabolism of the cells of the 

 imaginal discs preserves them from 

 these attacks. He thinks Kowalevsky is 

 piobably right in supposing that the 

 discs render themselves immime by 

 some poisonous secretion. 



It seems to me that the supposition of 

 a piotecting secretion is hardly neces- 

 sary to account for the phenomenon, 

 and it certainly would not account for 

 the preservation of those functional lar- 

 val tissues which aie not destroyed. 

 As soon as the larval life is over, in those 

 organs and tissues which then become 

 functionless and inactive, a healthy 

 metabolism would cease, and they would 

 begin to exhibit signs of weakness and 

 degeneracy. In this condition they 

 would be an easy prey to phagocytes or 

 disintegrating influences of any sort. 



