MOUTH PARTS OF THE SUBIMAGO AND IMAGO. 



Normal Muscle. A nomal muscle of the mouth-parts in longitudinal sec- 

 tion shows the dark bands relatively broad and deep blue staining. The light 

 bands are narrow, with Krause's membrane showing as a distinct dark line. The 

 nuclei are oval and lie just under the sarcolemma. Their chromatin is coarse, 

 dark purple staining, and lies chiefly about the periphery. In cross section Cohn- 

 lieim's areas are evident. 



Plasma Cells. In the body plasm from the first instar through imaginal 

 life there are five varieties of cells (PI. VI, fig. 87), which intergrade with no 

 sharp lines of demarcation : 



A. Little cells with a small amount of deep-blue staining, homogeneous 

 cytoplasm; nuclear chromatin evenly distributed, deep-purple staining, and coarsely 

 reticular. These cells show very slight ameboid movement. 



B. Cells 1.5 to 3 times the size of variety A, with a large amount of 

 homogeneous cytoplasm varying from dark to light blue ; nuclear chromatin deep- 

 purple staining, coarsely reticular and evenly distributed. These cells show mitotic 

 division and active ameboid movement. 



C. Cells 2 to 3 times the size of variety A ; with light blue staining vacuo- 

 lated cytoplasm ; nuclear chromatin finely reticular and light-purple to dark-blue 

 staining. Spindle forms are common. Ameboid movement is active. 



D. Cells 2 to 3 times the size of variety A ; with small pink staining areas 

 or a few pink granules ; cytoplasm light blue staining and vacuolated, similar to 

 variety C. Spindle forms are common. Ameboid movement is active. 



E. Cells 1 to 4 times the size of variety A ; with many deep-pink staining 

 granules ; cytoplasm staining a very light reticular blue, or not at all ; nucleus 

 hght purple, with coarsely reticular chromatin, and light staining ground sub- 

 stance. 



Smears made from the abdominal cavity of the nymph and subimago just 

 after molting show many degenerating plasma cells. The most common form is 

 vacuolated, with the nucleus crowded to the periphery (fig. 87, w). Some of 

 the vacuoles contain pink granules or diffuse pink areas. 



Degenerating Muscle. Xot all the muscles of the mouth-parts de- 

 generate simultaneously ; neither is an entire muscle at the same stage of degenera- 

 tion. This process is evident in portions of some of the mouth-part muscles of 

 Hexagenia recurz'ata five days before emergence. In all the specimens examined 

 cross-striations were visible in portions of the muscle just before transformation. 



Early in degeneration the dark bands stain lighter and irregularly (PI. 

 VI, fig. 86, I). The light bands increase in width, and Krause's membrane is no 

 longer evident. The muscle nuclei (n) are swollen and stain faintly. There is also 

 slight evidence of fibrillation. Then the muscle substance changes from blue to 



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