96 



CELL-DIVISION 



nucleus,! ^nd in such forms as CJiilomouas and Tmchclomonas, where 

 the <^ranules are permanently aggret^ated 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 



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Fig. 44. — Mitosis in the rhizoped Actinospluvrium. [Braukr.] 



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 ^////r*", consisting of double chromatin-granules. E. Early anaphase. E.G. Later ana- 

 phases. H. F^inal anaphase. /. Telophase; daughter-nucleus forming, chromatin in loop-shaped 

 threads; outside the nuclear membrane the centrosome, already divided, and the aster. J. 1-ater 

 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 sustaininp^ 

 Brauer's conclusion that the es.sential fact in the history of the chro- 

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



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

 in Achromatimn. 

 ■ '-^ For speculations on the historical origin of the centrosome, etc., see p. 315. 



