breed: metamorphosis of the muscles of a beetle. 351 



The course of events in the destructive changes of the contractile 

 substance is quite evident from these three stages. The muscle columns 

 break up into their primitive fibrillae, and these then undergo dissolution. 

 The sarcoplasm increases in amount during this process, but not enough 

 to balance the loss in volume caused by the dissolution of the fibrillae, so 

 that each fibre shrinks in actual volume. This is shown by a determi- 

 nation of the volume of the largest fibi'e of musculus metanoti (Plate 1, Fig- 

 ure 1, mt'nt.) in each of the three stages described. Of course there is a 

 chance for error in this determination, in that the muscle fibres vary in size 

 in different individuals ; but the ratios of the volumes in the three stages 

 will at least give an indication of the amount of shrinkage. The ratios 



-, ■ 1 1 stage i : ii : iii 



of the volumes are m the case determined very nearly, — r^^ — - — ^r- 



•^ volume 4 : o : 2 



From this it seems probable that not all of the material derived from the 



dissolution of the fibrillae is transformed immediately into sarcoplasm, 



but that some of it remains for a time in solution. It is suggested above 



that the agent which causes this dissolution is the sarcoplasm. There is 



no evidence of the action of leucocytes, either phagocytic or lyocytic, 



since they come into the neighborhood of the muscles only occasionally ; 



nor is there reason for supposing action on the part of other outside 



agents. 



During the whole period of these destructive changes the muscle 

 nuclei undergo frequent amitotic divisions. The larval nuclei (Plate 7, 

 Figure 34, nl.) before division are comparatively large, with usually a 

 single definite nucleolus. Figure 34 shows a nucleus dividing amitoti- 

 cally (nl.^) and three pairs of smaller nuclei (nl.^), the resultants of 

 such divisions. At pupation very few of the nuclei presenting the 

 characteristics of nl. are found, whereas very much elongated nuclei 

 (Plate 6, Figure 25, ??/.,* shows one that is comparatively short) are found 

 associated with strings of nuclei which have arisen from the division of 

 such elongated ones. Many of these nuclei no longer lie at the periphery 

 of a fibre, nor even at the periphery of one of the strands which have 

 arisen from the division of a fibre, but are deeply embedded in the 

 muscle substance (Figures 14, 27, 28). 



The sarcoplasm found at the surface of the larval fibres becomes lost 

 at an early stage, intermingling with the increasing amount of sarcoplasm 

 between the fibrillae. 



The only tissues, other than the muscular, which need to be considered 

 in this connection are the tracheae and the embryonic tracheal cells. 

 The tracheal endings on the muscles before any change takes place have 



