MODIFICATIONS OF MITOSIS 



65 



nuclear structure, which may have arisen through a condensation 

 or differentiation of the "achromatic" constituents. Noctilitca, the 

 diatoms, and Actinosp/icen/in seem to represent transitions to the 

 higher types. In the latter form Brauer discovered a distinct cen- 

 trosome lying in the late anaphase outside the nuclear membrane at 

 the centre of a small but distinct aster and soon dividing into two, 

 precisely as in higher forms (Fig. 31, /, J). This centrosome, how- 

 ever, as Brauer infers, lies within 

 the nucleus during the resting state 

 and the earlier stages of division, 

 and only migrates out into the 

 cytoplasm during the late ana- 

 phase, afterward returning to the 

 nucleus and lying in the "pole- 

 plate." In the diatoms Butschli 

 discovered an extra-nuclear centro- 

 some and attraction-sphere, and 

 Lauterborn has traced the forma- 

 tion of a central spindle from it. 

 This spindle, at first extra-nuclear, 

 is asserted to pass subsequently 

 into the interior of the nucleus. 



Noctilitca, finally, appears to 

 have attained the condition char- 

 acteristic of the higher forms. 

 Here, as Ishikawa has shown, the 

 cell contains a typical extra-nuclear 

 centrosome and attraction-sphere 

 lying in the cytoplasm, precisely 

 as in Ascaris (Fig. 30). By divi- 

 sion of centrosome and sphere a 

 typical central spindle is formed, 

 about which the nucleus wraps it- 

 self, and mitosis proceeds much as 



A 



';i-^-^:<;^^. 





B 



sp 



■■i?'-.->>-''--»'?5>. 



Pig- 30- — Mitosis in the Flagellate .V(7t//- 



A. Nucleus (fi) in the early prophase; 

 outside it the attraction-sphere [s), containing 

 two centrosomes (Ishikawa). B. The mitotic 

 figure; n. the nucleus, containing rod-shaped 

 chromosomes; s. attraction-sphere; s./>. ex- 

 tra-nuclear central spindle. (Drawn by G. N. 

 Calkins from one of his own preparations.) 



in the higher types, except that 



the nuclear membrane does not disappear. ^ 



Regarding the history of the chromatin the most thorough obser- 

 vations have been made by Schewiakoff in EiiglypJia and Brauer in 

 Actuwsphcerinin. In the former case a segmented spireme arises from 

 the resting reticulum, and long, rod-shaped chromosomes are formed, 

 which are stated to split lengthwise as in the usual forms of mitosis. 

 The nuclear membrane persists throughout, and the entire mitotic 



.AH of the essential features in this process, as described by Ishikawa, have been con- 

 lirmed by Calkins in the Columbia laboratory. 

 F 



