ORIGIN OF THE CENTROSOME. 28 1 



aster filaments are identical. Only in the muscle the size and 

 arrangement of the thickenings are somewhat more regular 

 than those in the aster filament. 



As has been already stated, the contractile filaments in the 

 muscle cell are arranged with perfect regularity, and are par- 

 allel with one another, passing through a common plane, — 

 "the Krause's membrane," or "Z." Suppose these fibrils, 

 instead of running parallel with one another, converge into one 

 common center {C, Fig. 7, III). The central ends of the fibrils, 

 instead of forming an elongated contraction band in the middle 

 plane of the cell, will then form a contraction sphere in the center 

 of it. In other words, the contraction sphere in the center of 

 the radial fibrils will be the centrosome ; and the varicosities 

 along the fibrils will correspond to the varicosities of the muscle 

 fibrils, and the whole system will constitute the aster. In the 

 muscle cell, the fibrils being arranged side by side, their con- 

 traction and expansion result in the shortening and lengthen- 

 ing of the whole cell along the longitudinal axis. In the aster- 

 bearing cell the contraction and expansion of the fibrils will 

 result in the movement of the cell-mass along the radii of the 

 aster. The principle involved in both cases is identical; the 

 difference in results is due to the dissimilar arrangement of 

 the cytoplasmic filaments. 



Such thickenings {M, Fig. 7) in the cytoplasmic fibrils are 

 called cytoniicrosomes ; when such cytomicrosomes attain a 

 more or less conspicuous dimension, or several of them fuse 

 into a common mass in the center of the aster, they give rise 

 to the centrosome ; when several microsomes belonging to the 

 parallel fibrils are arranged along the common plane, as in a 

 muscle cell, in the state of contraction, they give rise to what 

 is known as the contraction band (C). The case of a pigment 

 cell, in which the centrosome appears as a linear rod (Fig. 4, d), 

 suggests a close parallel to this contraction band. 



But the centrosome once formed in the center of the aster, 

 unlike the contraction band of the muscle cell, is apt to persist 

 for a long time; further, it seems to undergo some chemical 

 changes in certain cases, as is shown by the staining re- 

 agent, which is slightly different from that of the ordinary micro- 



