ORIGIN OF THE CENTROSOME. 



275 



Investigators who have studied this subject have generally 

 selected cells in which the centrosome appeared in a most con- 

 spicuous form; and naturally, as the main object at first was to 

 demonstrate its existence. It is interesting to notice that the 

 permanent-organ theory of the centrosome had its first origin 

 among those who studied the structure in its most conspicu- 

 ously developed form, as in the egg of Ascaris. 



But the possibility of discovering the affinity of the centro- 

 some to any other cell-constituent is rendered all the more 

 difficult, as long as our attention is directed only to those cells 

 in which this organ has reached its highest development. From 

 the standpoint of the derivation theory, however, such extreme 

 cases are just the ones to be avoided, for the theory presup- 

 poses an element in the cell which may be directly compared 



Fig. I. The egg of Unto complanata. C, centrosome : .V, nucleus, chromosomes not repre- 

 sented : Yk, yolk-granules. The centrosome is spherical in shape. 



with it; and it stands to reason that, if such an element really 

 exists, it cannot be a very conspicuous structure, or otherwise 

 the theory of the centrosome as a unique organ would never 

 have been proposed. 



These considerations, then, suggest inquiries along two 



lines : 



