403 



in qunntity ; karyosomes disappear and the nucleoli develop in 

 their place (fig. 102a and b). 



Although the adult muscles may be inserted upon the 

 cuticle direcftly, yet there can be no doubt that such insertions 

 were originally integumental cells. Leydig (1885) first put 

 forward this view, and it is held by Duboscq (1898), Hennequy 

 (1906), Janet (1907), and Perez (1910). Others have regarded 

 the fibrillae as fusing directly with the cuticle, but this view 

 seems scarcely tenable. 



If the muscles of young larvae be teased out, it is fre- 

 quently possible to observe the fibrils of the longitudinal 

 muscles communicating with the cytoplasm of a long process 

 from the flat integumental cells (fig. 100). These processes 

 show a considerable degree of chitinisation, and may appar- 

 ently chitinise fully before the end of larval life, thus explain- 

 ing the insertion of fibrillae upon a non-protoplasmic surface. 



The Dilators of the Pharynx. 



If a larva in its first instar be examined these muscles 

 can be observed in the last stage of development (fig. 47). 

 The muscles are formed, probably in late embryonic life, from 

 a mass of cells which fuse to form a syncytium. Generally 

 about four tO' five cells combine thus, though in some cases 

 as many as fourteen to sixteen (judging by the number of 

 nuclei) fuse. I have not examined these muscles earlier than 

 half-way through the first instar. At that stage the ''trans- 

 verse" striations are clearly seen, again in the form of spirals. 

 The nuclei all become concentrated in one place, and collecting 

 a certain amount of cytoplasm round them, form the large 

 swellings already mentioned. Each nucleus has a large 

 karyosome. 



The spiral striations do not extend on to the dilated 

 part of the cytoplasm ; they are confined to the essentially 

 contractile region of the muscle. Along this region fibrilla- 

 tion has been taking place, but is not yet, apparently, com- 

 plete, for the spirals extend outwards, upon otherwise 

 quite undifferentiated protoplasm (see fig. 47, at x). Here, 

 then, it seems that the (spiral) striations form first in the 

 contractile syncytium, and the longitudinal fibrilliation is 

 only secondarily developed. In the general body muscles 

 the opposite happens ; this appears to be the case also in 

 mammalian muscle. 



Sometimes a single cell of the syncytium may form a 

 number of distinct roots of the muscular portion of the 

 insertions. As in the body muscles, the muscle fibres are 

 always inserted upon the integumental cells, never directly 

 upon the cuticle. 



