423 



from now on each muscle is one great syncytial mass, corre- 

 sponding at first eight to an ordinary muscle in constitution, 

 but built up, in reality, in an entirely different manner; it is 

 a muscle built up of numerous fibres, and not a fibre which 

 consists of many fibrillae. 



In the thirty-six hour pupa a very curious thing is 

 now seen, the interpretation of which is difficult; each muscle 

 now presents a very faint striation, the striations being again 

 in the form of a double spiral. The striations never become 

 chromatic, as they usually do; nor do they correspond to 

 the striations that develop later in the individual fibres. 

 It will be seen later, when the muscle insertions are described, 

 that these muscles are already pulling on the body wall, 

 somewhat as they do in adult life, and perhaps the spiral 

 striations represent the direction of the strain within the 

 muscle, just as they do in muscles in which they are fully 

 differentiated; but since the "pull" which these muscles exert 

 at this time may be of only temporary duration, structural 

 differentiation does not follow. It is interesting to note that | / 

 the distance between successive spirals is almost exactly iden- I ** 

 tical with what is seen in other muscles, viz., about 8/x. / 



Finally, in the pupa of the fourth day the individual 

 fibres (sarcostyles) begin to show transverse striations (fig. 

 138) ; Krause's membranes, and the striations with Hensen's 

 line between them, are all clearly seen ; even the minute 

 tubules which Schafer describes appear to be visible; the 

 structures, however, are so exceedingly minute that at present 

 no further details can be given. The distance between suc- 

 cessive Krause's membranes is about 2|/x, so that an un- 

 differentiated spiral of the thirty-six hour pupa corresponds 

 to about three "striations" of the component fibres. 



Schafer was unable to observe any clearly defined stria- 

 tions in the muscle as a whole; there is no doubt, however, 

 that in Nasoma the striations of adjacent fibres are so dis- 

 posed as to present a true striation in the muscle as a whole ; 

 nor is it surprising to find that these striations are disposed 

 again in a double spiral. 



Schafer mentioned the occurrence of the nuclei within the 

 muscle, i.e., amongst the constituent fibres; there can be 

 no doubt that in Nasoma, and also in Calliphora, according 

 to the observations of Perez, the nuclei surround the muscle. 



The conclusions of Perez, in regard to the development 

 of the imaginal muscles, may be referred to here. This author 

 described the myoblasts in Calliphora as spreading themselves 

 over five pairs of thoracic muscles. The myoblasts, entering 

 these, lose their cytoplasm, and apparently grow at the 



