421 



therefore concerned indirectly in the reformation of the longi- 

 tudinal muscles of the thorax of the imago. 



The myoblasts divide mitotically (fig. 112); they are ! 

 rounded ovoid or hexagonal cells with large ''Yesicular" nuclei, i 

 Tliey measure 5j/x to 6^ in width, and have not appreciably ; 

 altered in size throughout larval life. H/jjCvi ^^i^*^^^^*^ J 



The myoblasts jever al hours later'begin to penetrate the 

 tough sarcolemma and work their way into the degenerate 

 muscle substance (fig. 129) ; others follow, and in the larva 

 eight hours after def aecation the whole muscle becomes riddled 

 with myoblasts, all nourishing themselves, apparently, on ^^ 

 the disintegrated larval muscle^ They never lose their cyto- 

 plasm, such as Perez describes in Calliphora. 



Any leucocytes which may be close by gorge themselves 

 upon the dead muscle (fig. 129), but the myoblasts seem to 

 proliferate so rapidly that before the few leucocytes, which 

 may be present, have had time to depart they become en- 

 tangled in the mass of myoblasts, and rapidly degenerate 

 there (fig. 128). They are frequently seen in the early pupa 

 — long after the last remnants of the larval muscles have dis- * 

 appeared — as spherical bodies, a little larger than the 

 myoblasts, in which several large heavily staining globules 

 are present (y in fig. 135), and in which the leucocyte nucleus 

 may still sometimes be observed. The presence of the 

 embryonic cells appears to bring about their precocious dis- 

 integration. It is these disintegrating leucocytes, I believe, 

 that Perez has taken for the nuclei of the larval muscles, 

 undergoing "chromatolitic disintegration'." In Nasonia their 

 nature is unmistakable; they do not become apparent till about 

 sixteen hours after the larval muscles have disappeared. 



Meanwhile the myoblasts have absorbed more and more 

 of the larval muscles, and so extraordinarily rapid is the 

 process that in the larva twelve hours after def aecation no 

 trace of the larval muscles remains ; in their place there occurs 

 now a pair of bands of myoblasts lying some distance below 

 the integument, on either side of the midline (figs. 130, 132). 

 The two columns of myoblasts are at first unexpectedly small, 

 measuring only 55fjL in breadth, 14 to 15/x in thickness; they 

 extend from the rear of the mesothoracic segment to a con- 

 siderable distance into what was the prothoracic segment of 

 the larva. 



But already . before the end of larval life a series of 

 remarkable processes begins, which transforms these two strips 

 of embryonic cells into the five pairs of great longitudinal 

 thoracic muscles of the imago. In the larva, shortly before 

 pupation, certain of the cells of these two myoblastic bands 



