282 BIOLOGICAL LECTURES. 



some of the cytoplasm. Moreover, the centrosome may sever its 

 connection with the radial rays, as indicated by the formation 

 of a clear space around it, and finally, the centrosome may 

 become wholly bereft of its rays, and stands alone naked in 

 the general mass of the cytoplasm. But these are secondary 

 phenomena which come into play after the centrosome has 

 once been definitely formed. Too much emphasis laid on 

 these secondary features, which are induced after the centro- 

 some has once been formed, is liable to lead one to lose sight of 

 the primary process which is directly concerned in its formation. 



As I have maintained elsewhere, a satisfactory solution of 

 the problem of the centrosome depends on the explanation of 

 the relation existing between the cytomicrosomes and their 

 connecting cytoplasmic filaments. If the preceding interpre- 

 tation of the origin of the centrosome is true, it seems that 

 the substance of the cytoplasmic filament and that of its micro- 

 some stand in genetic relationship, exactly in the same way as 

 that which takes place in the contraction and expansion of the 

 muscle fibrils. In short, the history of the filament and its 

 microsome runs in a cycle. The microsome may be converted 

 into the filament under one condition, and the filament in turn, 

 under another condition, may give rise to a microsome at the 

 expense of its material. They may, therefore, be considered as 

 two alternating phases of one and the same cytoplasmic substance. 



If this view be a true one, the centrosome is simply a modi- 

 fied portion of the cytoplasm, and not a permanent organ. 

 According to this view, the centrosome is no more a perma- 

 nent organ of the cell than is the contraction band in the stri- 

 ated muscle cell. Only the centrosome once formed in the 

 focal point of the several fibrils, as has been already stated, is 

 apt to be more persistent than the contraction band. 



And further, just as the contraction band can go back to 

 the state of original filamentous substance during the expan- 

 sion of the muscle cell, so the centrosome can give rise to the 

 cytoplasmic filament, as may be seen in the caryokinetic pro- 

 cess in certain cells, where the clear, smooth, cytoplasmic fila- 

 ments are seen coming out from the centrosome. This is 

 particularly well seen in those cases where the centrosome 



