MODIFICATIONS OF MITOSIS 



nuclear structure, which may have arisen through a condensation 

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

 diatoms, and Actinosphariun seem to represent transitions to the 

 higher tyg.es. 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. 3 1,/, 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 Biitschli 

 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. 



Noctilucct) 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- 



j i 



sion of centrosome and sphere a outside it lhe attracdon _ spher e 



typical central spindle is formed, two cjsntrosomes( Ishikawa). B. The mitotic 



about which the nucleus wraps it- ^^^SS^S^L 



Self, and mitosis proceeds much as tra-nuclear central spindle. (Drawn by G. N. 

 in the higher types, except that Calkins from one of his own pteparatib 



the nuclear membrane does not disappear. a 



Regarding the history of the chromatin the most thorough obser- 

 vations have been made by Schewiakoff in Euglypha and Brauer in 

 Actinosph&rium. 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 



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

 firmed by Calkins in the Columbia laboratory. 



F 



B 



Fig. 30. Mitosis in the Flagellate Nocti- 

 luca. 



A. Nucleus () in the early prophase ; 

 , containing 



