ORIGIN OF THE CENTROSOME. 287 



staining material; in another cell this sphere is represented by 

 a greater or less number of discrete granules which bear the 

 closest resemblance to the ordinary microsome of the cyto- 

 plasm; in still another, it assumes the shape of a linear rod, 

 reminding one of the contraction band of the striated muscle 

 cell. In a pigment cell the centrosome may even assume the 

 reticular framework, consisting of strands of deeply staining 

 cytoplasmic material. Indeed, to use the terfn centrosome in 

 the sense it was originally intended, appears hardly appropriate 

 to cover all these cases. And any one who attempts to ex- 

 plain the nature of the centrosome must not confine himself 

 to the consideration of the spherical type, with which we are 

 now most familiar, but must take in all other forms under some 

 common point of view. 



On the derivation theory, as explained in the present paper, 

 such polymorphism of the centrosome is full of significance. 

 If, as explained already, the centrosome is the modified cyto- 

 plasm, which takes divers shapes in correlation with some defi- 

 nite motion of the protoplasm, such diversity of its forms in 

 different cells is not at all surprising. They are the structures 

 which originated independently in different cells, but having 

 been evolved in correlation with the same function in all cases, 

 careful researches disclose some curious similarity even amidst 

 the features of great anatomical divergence. 



On this derivation theory, also, the absence of the centro- 

 some in the fixed cell becomes intelligible. If, as has been 

 already pointed out, such fixed cells show any decided phe- 

 nomena of intracellular movement as caryokinesis, the centro- 

 some is again reconstructed from the ordinary cytoplasm ; the 

 division over, the rearrangement of the cytoplasm comes in, 

 and even the centrosome, though a more persistent structure 

 than the spindle, becomes eventually merged in the general 

 cytoplasm of the "resting" cell. 



If this view seems to detract from the dignity which the 

 centrosome would have as a permanent organ of the cell, it 

 may be said, on the other hand, to emphasize a certain endow- 

 ment of the cytoplasm which has not been fully recognized in 

 connection with the problem of the origin of the centrosome. 



