V 



412 



fibrillation, to others which undergo total disorganization, 

 but fail to cast their contents into the body cavity. 



In the larva at about the time of defaecation the myo- 

 blasts, which may often be in the form of spindle-shaped 

 cells, proliferate rapidly (fig. 117), and an occasional myoblast 

 may be observed entering the degenerate- muscle. The rupture 

 thus made serves for the entrance of the myoblasts, and soon 

 several groups of myoblasts, now quite round, may be 

 observed, one behind the other, all lying in the path cleared 

 by the first myoblast (fig. 121). The cytoplasm of each 

 myoblast is, contrary to the observations of Perez, usually 

 clearly visible, lying within a clear space which it has 

 excavated out of tlie muscle substance and the former contents 

 of which it has apparently absorbed (fig. 121). Frequently, 

 however, the cytoplasm of the myoblasts is so similar to that 

 \ of the disintegrated muscle substance that its limits cannot 

 V be recognized. More and more myoblasts penetrate the 

 muscle fibre till, in the larva eight hours later, the whole 

 muscle is riddled with embryonic cells; the sarcolemma seems 

 to be absorbed also. During the remainder of larval life the 

 myoblasts, after absorbing the remnants of the granulated 

 larval muscle, arrange themselves in several columns of cells ; 

 the cells may be slightly spindle-shaped, at other times brick- 

 shaped, and each column is to be considered the equivalent 

 of one developing musclo fibre such as I have described in the 

 abdomen ; the pharyngeal dilator muscles, in other words, are 

 multifibrous structures, of much greater complexity than the 

 ordinary abdominal muscles. By this process six pairs of 

 pharyngeal muscles of the adult are laid down ; two other 

 pairs are developed from myoblasts which appear to grow 

 quite independently of the larval muscles. At any rate, eight 

 pairs of muscles are to be observed in the newly formed pupa 

 (see fig. 154). 



In the fresh pupa the muscles begin to differentiate. 

 Each column of cells becomes a long columnar syncytium, 

 just as occurs in the abdominal muscles, so that in the fresh 

 pupa the developing muscle consists of a number of syncytial 

 columns packed close together (fig. 122). Each column then 

 f undergoes longitudinal fibrillation, and the whole muscle, 

 ) losing all indication of the individual columns, becomes a 



i uniform mass of longitudinal fibrillae. The whole process 



goes on very rapidly, and all stages from a non-syncytial mass 

 to a true fibrillated mass can be observed in the fresh pupa. 

 Even at this time distinct indications of striations can be 

 observed, each fibril breaking up into alternate elements, one 

 of which stains feebly with haematoxylin, the other with 

 eosin. No distinct Krause's membrane in the individual 



