584 VERA DANCHAKOFF 



direction opposite to the former. Is the growth of the spindle 

 influenced by the achromatic part of the nucleus or by the chro- 

 mosomes, which at this stage are in full development? The at- 

 tachment of the spindle fibers to the chromosomes seems to speak 

 (McClendon, McClung) for the existence of a pronounced mutual 

 attraction between these structures. The appearance of the 

 spindle within the nucleus influences the distribution of the 

 chromosomes. These latter are pushed by the growing spindle 

 to the equator of the nucleus, where they become situated at one 

 level, forming the equatorial plate of the mitotic figure. At this 

 stage the differentiation of the chromatin into the chromosomes 

 and of the plastosomes into the spindle attains its culminating 

 point. 



Both in cleavage and in division of somatic cells a supreme 

 importance has been attributed to the centrosome. Boveri con- 

 siders the centrosome as an organ, from which the impulse to 

 cell division arises. Though many details of cell division are 

 now known, the principle of this elementary process is not yet 

 recognized. With the discovery of the centrosome a ray of light 

 seemed to penetrate the field of cell division studies. But un- 

 fortunately this ray was soon taken for the source of the light, 

 for the prime and final cause of cell division. To Boveri the 

 centrosome appeared as a self propagating cell organ which sup- 

 plied the dividing cell with mechanical regularity. This view 

 found support in the papers of many biologists. And though 

 Boveri himself does not regard the centrosomes of equal im- 

 portance with the chromosomes, some of his followers went, 

 in their deduction, much further than he. 



For a while the importance of the centrosome was strongly 

 emphasized, but in the face of cumulative adverse evidence it 

 can no more be regarded as a permanent cell organ (Lillie, Child, 

 Wilson, Morgan, Hertwig, Klinkowstrom) . My own results 

 strengthen this conclusion. 



The centrosomes first appear in my preparations at the poles 

 of the nucleus. Their differentiation proceeds locally, without 

 any relation to the centrosomes of the spermatozoon or to the 

 primary single radiation around the segmentation nucleus. The 



