409 



observe the muscles in their embryonic state has resulted 

 in a considerable misinterpretation of the structure of the 

 mature organs. 



The adult muscles all arise from mesodermal cells, the 

 myoblasts, which are recognizable in the earliest larvae. The 

 assertion of de Vaney (1902) that these cells are hypodermal 

 in origin, is quite erroneous, and the opinions of Kowalevsky 

 (1887), Berlese (1901), Henneguy (1904), Karawaiew 

 (1898), Perez (1910), and finally of Poyarkoff (1910), that 

 they are essentially mesodermic cells are easily verified in 

 Nasonia. 



(1) The Superficial Longitudinal Abdominal Muscles. 



As the most direct development of adult muscles occurs 

 in the superficial abdominal muscles, it is best to consider 

 these first. 



In the fresh pupa, the longitudinal abdominal muscles 

 begin to degenerate. After losing their striations the fibrillae 

 cluster together in the middle of the muscle, while the inter- 

 stitial substance, which in life separates them, becomes forced 

 to the periphery of the muscle fibre, appearing here as a 

 granular fluid, after showing fatty globules (fig, 107). At 

 other times the whole muscle fibre, not merely its interstitial 

 substance, may undergo granular degeneration, and the 

 chromatic material, breaking out of the nucleus, may scatter 

 itself as fine granules amongst the degenerate cytoplasm. The 

 sarcolemma remains intact (fig. 108). Some of these muscles 

 undergo phagocytic destruction, as above described. It is to 

 the remainder that I refer here. 



The myoblasts now become active. During larval life 

 these have been lying, as small embryonic cells, h[i to 6/x in 

 diameter, scattered in the body cavity close to the muscles. 

 They now begin to multiply, mitotically it seems, and, pene- 

 trating ^the sarcolemma, lie in the degenerat-ed muscle 

 cytoplasm (figs. 106, 108, 109), where they move about by 

 an^oeboid action (fig. 107). 



Within the muscle fibre these cells multiply, and grow at 

 the expense of the degenerate larval muscle substance. In 

 those muscles where there has occurred a total cytoplasmic 

 degeneration their task seems comparatively easy; but in 

 those muscles where the fibrillae have failed to disintegrate 

 they at first confine their attention to the granular inter- 

 stitial substance. Eventually, however, the whole larval 

 muscle (fibre) disappears, including even the sarcolemma, and 

 the myoblasts are seen in its place. The cytoplasm of the 

 myoblasts is always clearly seen, and pseudopodia are often 

 visible (fig. 107); but whether the myoblasts absorb the 



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