9 <5 



CELL-DIVISION 



nucleus, 1 and in such forms as CJiilomonas and 'Traclielomonas, where 

 the granules are permanently aggregated about a central body. Too 

 little is known of the facts to justify a very positive statement; but 

 on the whole they point toward the conclusion that in the simplest 



Fig. 44. Mitosis in the rhizoped ActinospkcBriuin. [BRAUER.J 



A. Nucleus and surrounding structures in the early prophase ; above and below the reticular 

 nucleus lie the semilunar " pole-plates," and outside these the cytoplasmic masses in which the 

 asters afterward develop. B. Later stage of the nucleus. D. Mitotic figure in the metaphase, 

 showing equatorial plate, intra-nuclear spindle, and pole-plates (p.p.). C. Equatorial plate, 

 viewed en face, consisting of double chromatin-granuies. E. Early anaphase. F. G. Later ana- 

 phases. //. Final anaphase. /. Telophase ; daughter-nucleus forming, chromatin in loop-shaped 

 threads ; outside the nuclear membrane the centrosome, already divided, and the aster. J. Later 

 stage; the daughter-nucleus established; divergence of the centrosomes. Beyond this point the 

 centrosomes have not been followed. 



types of mitosis no true chromosome-formation occurs, thus sustaining 

 Brauer's conclusion that the essential fact in the history of the chro- 

 matin in mitosis is the fission of the individual granules. 2 



1 The fission of the individual granules is carefully described and figured by Schewiakoff 

 in Achromatiitm. 



' 2 For speculations on the historical origin of the centrosome, etc., see p. 315. 



