90 



CELL-DIVISION 



extranuclear body, as in most Metazoa {Heliosoa, Noctiluca, Para- 

 moeba\ 



5. In a few forms having a scattered nucleus the chromatin-gran- 

 ules are only collected about the apparently persistent sphere or 

 centrosome at the time of its division, and afterward scatter through 

 the cell, leaving the sphere lying in the general cell-substance 

 {TctramitHs). 



6. The arrangement of the chromatin-granules to form chromo- 

 somes appears to be of a secondary importance as compared with 



f^K'M 



A B CD 



Fig- 39. — Mitosis in the rhizopod, Euglypha. [SCHEWIAKOFF.] 

 In this form the body is surrounded by a firm shell which prevents direct constriction of the 

 cell-body. The latter therefore divides by a process of budding from the opening of the shell 

 (the initial phase shown at .4) ; the nucleus meanwhile divides, and one of the daughter-nuclei 

 afterward wanders out into the bud. 



A. Early prophase ; nucleus near lower end containing a nucleolus and numerous chromo- 

 somes. B. Equatorial plate and spindle formed inside the nucleus ; pole-bodies or pole-plates 

 {i.e. attraction-spheres or centrosomes) at the spindle-poles. C. Metaphase. D. Late ana- 

 phase, spindle dividing; after division of the spindle the outer nucleus wanders out into the bud. 



higher forms, and the essential feature in nuclear division appears to 

 be the fission of the individual granules. 



We may first consider especially the achromatic figure. The basis 

 of our knowledge in this field was laid by Richard Hertwig through his 

 studies on an infusorian, SpirocJiona ( ''J J), and a rhizopod, Actiito- 

 spJicBriinn ('84). In both these forms a typical spindle and equatorial 

 plate are formed inside tJic nuclear membrane by a direct transfor- 

 mation of the nuclear substance. In SpirocJiona (Fig. 38, A-C) a 



