478 THEOPHILUS S. PAINTER 



THE CENTROSOME 



The present work throws no new light upon the origin and 

 nature of the centrosome, but certain phases in the behavior of 

 the division mechanism call for special mention here. 



Normally, the monaster condition lasts for a very short time 

 in the division cycle, only so long as the centrioles are 

 undivided, according to Boveri (Zellenstudien IV). In mon- 

 aster eggs this period may be prolonged through a whole divi- 

 sion cycle without seriously disturbing the division mechanism. 

 We are totally unable to suggest the cause of this delay in this 

 division (or the separation of the centrioles), but it seems clear 

 that the abnormal condition may be overcome at any period in 

 the division cycle. Thus a few eggs recover before the controls 

 have divided. In others the centrioles divide and separate when 

 the controls are preparing for the second division. In others 

 the whole division cycle may be passed through, and this is 

 what occurs in the majority of the eggs. Or finally the separa- 

 tion may be suppressed until the third or fourth division cycle 

 has been passed through; or, even permanently. 



The suppression of one division does not affect the centrioles 

 injuriously, in the majority of the eggs, but frequently in eggs 

 which have passed through several cycles in the monocentric 

 condition polyasters suddenly appear. How are we to ex- 

 plain such cases? Two possibilities present themselves. If 

 we assume that centrioles be at the focal point of the rays, 

 either the centrioles have divided at each division cycle but have 

 failed to separate, a view for which I have no evidence to sup- 

 port, or, as the inhibiting factor is removed, the centriole have 

 been excited to rapid division. The second effect may be ex- 

 perimentally obtained by treating eggs with various poisons. 



Another feature which should be mentioned is the increased 

 size of the asters in those eggs which have passed through one 

 division cycle and have then formed amphiasters. The ap- 

 pended figures Ki and K 2 show the size of the normal asters 

 K 2 (in the first division) and the spindle of an egg with a mon- 

 aster history K lt The difference in size at the same phase of 



