18 



PSYCHE. 



These observations of Weismann 

 opened a new field of investigation in 

 the development of insects. This has 

 been entered by several eminent inves- 

 tigators of whom Ganin, Ktinckel d'Her- 

 culais, Viallanes, Kowalevsky, and Van 

 Rees have been the most snccessful. 

 Ganin * in 1S76 demonstrated the pres- 

 ence of many other imaginal discs than 

 those which Weismann described. 

 Weismann supposed that the hypoder- 

 mis of the larval abdomen went di- 

 rectly with some modifications to form 

 that of the imago. Ganin showed, now, 

 that in the hypodermis of each of the 

 eight abdominal segments are four cell- 

 islands, two dorsal and two ventral, 

 which resemble in every respect tlie 

 tissue of the imaginal discs ; that they 

 are in fact imaginal discs and are des- 

 tined to form the starting points for the 

 growth of the imaginal abdominal hyp- 

 odermis. Ganin also discovered similar 

 discs in the epithelium of the midgut 

 whose fate it is to form, in the same 

 way, the imaginal midgut, and also 

 the important fact that each imaginal 

 disc in the larva is made up of two 

 kinds of embryonic tissue, ectoderm 

 and mesoderm or mesenchym. In 

 1875 Kilnckel d'Herculais f found in 

 the last abdominal segment two pairs 

 of imaginal discs of the external geni- 

 tal organs. In 1SS3 Metschnikofl' % 



* 1. c. p. 38&. 



t Klinckel d'Herculais. Reche.clies sur rorgainsalioii 

 ie developpement des Voliicelies. Paris, 1S75, P- H?- 



J Metsclinikoff. Untersucliuiig iiber intraccelliilare Ve 

 dauung bei Wirbellosentieren. Arb. a. d. Zool. Inst. 

 Wien. 5 Bd. iSSj.p. i^i. 



published the first of his epoch-mak- 

 ing observations on the destruction of 

 tissues in certain invertebrates by leu- 

 cocytes or as he called them phagocytes. 

 He discussed Ganin's paper and espe- 

 cially his statement that during the 

 histolysis of the pupal muscid the larval 

 organs are destroyed by amoeboid mes- 

 oderm cells. These cells he suggests 

 are none other than phagocytes. In 

 1SS4 Van Rees § and in 1S85 Kowalev- 

 sky II proved the correctness of this posi- 

 tion ; they showed that the process of 

 histolysis is the tearing down and diges- 

 tion of the functional larval tissues by 

 ]jhagocytes and the huildingup of imag- 

 inal tissues from imaginal discs. 



In 1 888 Van Rees f published liis ex- 

 tensive paper on the post-embryonic de- 

 velopment of muscids, and completed 

 our knowledge of this phenomenon. 

 He showed that when the muscid ian 

 larva has entered upon the pupal stage, 

 histolysis is inaugurated by the destruc- 

 tion of the larval muscles, they becom- 

 ing iiufunctional directly after pupation 

 antl a natural prey to the phagocytes. 

 Soon the thoracic hypodermis and the 

 inner organs are attacked, and at the 

 same time the imaginal discs begin to 

 grow and widen out to supjjly the place 

 of the tissues whicii are being destroyed. 

 The continuity of the hypodermis and 



§ Van Rees. Over inlra-cellulairespijsverteeriug en over 

 de beteckenisder witte bloedlichampjes. Slaandblad voor 

 Naturwetenscliappen. Jaarg. 11, 1SS4. 



II Kowalevsky. Beitrage z. nachembryonale Entwirklung 

 d. Miisciden. Zool. Anz, 8 Bd. iSSs.p. 98. 



^ Van Rees. Beitrage 2. Kenntniss d. inneren Meta- 

 morphose V. Musca vomitoria. Zool. Jahrb. Abth. t. .Anat. 

 u. Ontog. J. Bd. 18SX, p. 1. 



