MUSCULATURE. 381 



closely crowded together, encircle the anal aperture (Fig. 189, tins) 

 and take part in the formation of the proctodaeum, yielding the 

 rudiments of the rectal sac and the rectal papillae. To this segment 

 apparently also belong the two pairs of imaginal genital rudiments 

 (rudiments of the external genital organs) which were demonstrated 

 by Kunckel d'Herculais (No. 113) in Volucella. 



It should be mentioned that a cell-accumulation representing the permanent 

 mesoderm (Fig. 188 C, m) is found on the inner surface of the abdominal 

 imaginal discs as on that of the thoracic discs, this accumulation being the 

 starting-point for the development of the mesodermal structures of the abdomen. 

 Kowalevsky, as above mentioned (p. 375), has traced back the origin of this 

 mesoderm-accumulation to the so-called wandering cells (Fig. 188 A, w), while 

 earlier authors were inclined to derive them through delamination from the 

 ectoderm of the imaginal discs. 



The newly-formed hypodermis extends very rapidly over the 

 surface of the body, so that the areas of bypodermis originating 

 from the different imaginal discs soon flow together. While this 

 perfecting of the permanent hypodermis is taking place, that of 

 the larva is finally destroyed by the phagocytes. 



Musculature. 



The greater part (or the whole mass) of the larval musculature 

 undergoes a process of disintegration by means of phagocytes 

 precisely like that described above in connection with the larval 

 hypodermis ; the disintegration of the muscles is, indeed, the first 

 process to take place in the pupa. The muscles of the most anterior 

 segments of the body disintegrate first ; and the superficial layers 

 are affected before the deeper ones. 



The disintegration of the larval muscles is brought about in the 

 following way. A large number of amoeboid blood-corpuscles, which 

 have collected on the surface of the muscle-bundle, penetrate through 

 the sarcolemma and wander into the interior of the muscle-substance, 

 pressing into fissures which develop in it. It often appears as if the 

 muscle-substance is actually cut out in the parts corresponding to 

 the processes of the phagocytes which extend into it. The muscle, 

 in this way, breaks up into a number of particles which soon become 

 rounded and are immediately swallowed by the phagocytes. The 

 muscles are thus transformed into a great accumulation of granidar 

 spheres, which finally shift apart and become scattered in the body- 

 cavity of the pupa. The muscle-nuclei are digested and assimilated 

 by the phagocytes in the same way as the muscle-substance. 



