No. 4-] METAMORPHOSIS OF THE FLAG WEEITL. 189 



During growth the fat is being stored in undifferentiated 

 mesodermal cells which are free within the body cavity. These 

 cells become greatly distended 

 with the fat globules, which come 

 to fill great interstices in the pro- 

 toplasm toward the cell periphery. 

 That each nucleus retains its vi- 

 tality notwithstanding, is shown 

 by its staining reactions, and by 

 its retention of a central mass 

 of protoplasm about itself, from 

 which the peripheral strands that 

 encircle the fat globules proceed. 

 This is certainly not typical fatty 

 degeneration ; it seems to me 

 much more properly considered 

 to be partial anabolism, affected 

 by these cells in their rapid elab- 

 oration of hydrocarbons during 

 the transient period of abundant 



food supply. This view is corrob- 

 orated by their later history. They 

 do not (at least a majority of them 

 do not) die with the dissolution of 

 the fat. Nothing is plainer while 

 one is watching the disintegration 

 of the fat masses than that the 

 nuclei contained therein show none 

 of the usual signs of necrobiosis. 

 Here and there will be seen a 

 nucleus which, together with its 

 enveloping coat of protoplasm, 

 seems to be slipping itself free 

 from its aforetime accumulati"ii 

 of fat. Furthermore, these nuclei 

 thus isolated can be seen associat- 

 ing themselves with the developing 

 muscle rudiments, and, apparently, 



FIG. 8. Partial cross-section of a larva, 

 nearing pupation. >, fore wing; /.mid- 

 dle leg ; m, muscles ; _/", fat ; dv, dorsal 

 vessel ; k, alimentary canal ; e, digestive 

 epithelium, ready for dissolution ; ti, 

 nerve cord; o, o, a, areas of first disinte- 

 gration of the fat masses. 



B 



FIG. 9. The development of scales and of 

 muscle fibers. A , a bit of a longitudinal 

 section of the femur of a young pupa; 

 t, developing trachea ; .y, developing ten- 

 don (flexor tibiae) ; m, developing mus- 

 cle fibers; f, disintegrating fat; c, a 

 nucleus belonging to the fat mass isolat- 

 ing itself from the same ; b, basement 

 membrane ; tc, developing scales, in the 

 midst of ordinary hypodermis. B, a bit 

 of the body wall from a newly trans- 

 formed imago, lettered as in A . 



