350 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



division being completed at a stage when the wings have grown so 

 large that they begin to be crumpled and folded. Figure 14 (Plate 6) 

 and Figure 34 (Plate 7) show, respectively, cross and longitudinal sec- 

 tions in which this division has been partially accomplished. Figure 14 

 shows the cross section of six angular strands, the larger of which again 

 divide to form the usual eight or nine fibres of this muscle in the imago 

 (Figure 15), The rounding of these more or less angular strands into 

 the cylindrical form of a muscle fibre takes place in the very young 

 pupa. 



At an early stage in the division of the fibre, the sarcolemma is broken 

 up and soon disappears. 



The changes in the finer structure of the muscle substance during the 

 time in which the fibres undergo this division are very noticeable. These 

 changes are illustrated by a series of drawings magnified 1,600 diameters, 

 in which both cross and longitudinal sections are shown at three differ- 

 ent stages of the resting larva. Stage one (Figures 23, 26, Plate 6) 

 represents the condition before any change has taken place. Cohnheim's 

 areas (aa. Cohn.) are very plainly shown in the cross section, while the 

 longitudinal section shows both longitudinal fibrillation and cross 

 striations. 



Stage two (Figures 24, 27) is from a resting larva several days before 

 pupation. The figures are drawn from muscles which correspond in 

 their stages of development with those shown in Figures 14 (Plate 6) and 

 34 (Plate 7). In the figures at the higher magnification (Figures 24, 27) 

 it is seen that the muscle columns have partially separated into their 

 primitive fibrillae, Cohnheim's areas appearing in only a few places. 

 The cross striation has disappeared entirely, whereas the longitudinal 

 fibrillation shows nearly as plainly as before. The sarcoplasm between 

 the fibrillae has meanwhile increased in amount and now begins to take 

 a stain with thionin, a characteristic of the cytoplasm of all actively 

 growing tissues. This is a strong reason for believing that the sarco- 

 plasm is itself in an active metabolic condition, and therefore the agent 

 which is causing the solution of the fibrillae. 



Figures 25 and 28, which represent stage three, are drawn from a 

 series of sections of a larva which would have pupated in a few hours. 

 These figures show only a finely granular sarcoplasm, in which there is 

 no trace of the fibrillae of the previous stage, not even a suggestion of 

 longitudinal fibrillation remaining. The muscle as a whole appears still 

 more deeply stained than before, since none of the non-staining fibrillae 

 remain. 



