352 bulletin: museum of comparative zoology. 



been described. Immediately on the division of the muscle into strands, 

 the cells of these finer tracheoles begin very rapid mitotic division. 

 Cells in various stages of division (cl. init.) are to be found in nearly 

 every section of a muscle in a stage similar to Figure 14 (Plate 6) and 

 Figure 34 (Plate 7). Most of the new cells so formed become either 

 actually or apparently detached from the tracheoles, and penetrate into 

 the fissures between the muscle strands (cl. tr.). Some, however, re- 

 main connected with the tracheae and show tracheoles, running through 

 their cytoplasm (Figure 14, cl. tr.^). Especially in longitudinal sections 

 (Figure 34, cl. tr.) they show long processes, which frequently connect 

 with each other. These processes cause the cells to be of irregular forms, 

 the spindle form being, however, the most frequent. The cytoplasm 

 stains so deeply in thionin that the limits of the nuclei ai-e in many 

 cases difficult to determine. 



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foO So °of >>°0-||o<, O OOO^ 



o ° o^ ■" " 6 



Fig. a. 



Other considerations than those mentioned above point to the origin 

 of these cells from the cells of the walls of the tracheae. Figure A is 

 a projection of the nuclei of the tracheal cells (represented by the small 

 oval outlines) on an optical longitudinal section of the largest of the fibres 

 of musculus nietanoti (Plate 1, Figure 1, mfnt.) to show the positions 

 and numbers of these cells. The particular fibre chosen for this recon- 

 struction was in an early stage of its metamorphosis, the reconstruction 

 being made from a series of cross sections. similar to Figure 14 (Plate 6). 

 From the textfigure it is seen that near the places where the tracheae 

 join the fibre, tracheal cells are much more numerous than elsewhere, 

 and that they are distributed in just such positions as would be expected 

 if they were being formed from the intracellular tracheoles which arise 

 from the tracheae. This uneven distribution of the tracheal cells can 

 scarcely be explained by assuming an origin of these cells from nuclei of 

 the muscle fibre or from leucocytes. Mitosis is found in the cells of 



