4U 



They are distinguishable from the leucocytes on account of 

 their smaller size (about 6/x) and the clearness of their cyto- 

 plasm, which is quite devoid of vacuoles. 



During larval life these cells — the myoblasts of the future 

 head muscles — proliferate, but do not appreciably change 

 their size or appearance. Whether proliferation is confined 

 to the last stages of larval life, or whether it occurs gradually 

 throughout larval life, or, lastly, whether it occurs only at 

 the time of moulting, I have not observed. At the time of 

 defaecation, however, the myoblasts have proliferated greatly, 

 and still dividing mitotically, grow upwards and backwards 

 behind the brain as two slender columns of cells (figs. 91, 

 154) ; in the larva eight hours later they have crept right 

 up the back of the transforming head, and finally reached 

 the dorsal surface. The cells in the lower portions of the 

 columns have consolidated themselves, and now form a well- 

 defined rod. Tliose at the growing ends are loosely arranged 

 and generally long and ''spindle-shaped." Sometimes they 

 are exceedingly long, and apparently represent the cells which 

 both Breed and Anglas mistook for tracheoblasts. In growing 

 upwards they move along, and support themselves upon, the 

 degenerate larval tracheoles (fig. 91). 



In the twelve-hour larva these spindle-shaped cells have 

 all adopted the shape characteristic of the other cells of the 

 columns; further cell proliferation results in a thickening of 

 the columns. 



Although the columns have supported themselves, as 

 they grew upwards, upon the great larval tracheoles, they 

 soon stand quite independent of these. This appears to occur 

 at the time when the most dorsal cells have fixed themselves 

 to the ectoderm of the apex of the head. In the larva eight 

 hours before pupation the columns have become intimately 

 associated with these ectodermal cells. The remainder of the 

 development of the head muscles is intimately associated with 

 that of these muscle insertions. 



In the pupa in the first day of its existence the cells of 

 the two great columns have grouped themselves into a number 

 of secondary columns, by the regular arrangement of successive 

 cells one above the other. There are thus formed, still within 

 the limits of the original columns, numerous secondary 

 columns each one cell in thickness ; each of these columns 

 will become a single muscle (fibre) of the head. 



The dorsal extremities of the two columns, it would 

 seem, begin to spread out a little and meet the processes from 

 adjacent ectodermal cells — the future muscle insertions. 

 These are at first quite long, and even in the larva eight 

 hours before pupation may be observed converging from a 



