MODIFICATIONS OF MITOSIS 



63 



The essential features of the process appear to be here of the same 

 nature as in the higher types, but show a series of minor modifications 

 that indicate the origin of mitotic division from a simpler type. 

 Four of these modifications are of especial importance, viz. : — 



( 1 ) The centrosome or its equivalent lies as a rule inside the 

 nucleus, thus reversing the rule in higher forms. 



(2) The nuclear membrane as a rule remains intact and does not 

 disappear at any stage. 



B 



D 



Fig. 28. — Mitosis in the rhizopod, Eiiglypha. [Schfaviakoff.] 

 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 A) ; the nucleus meanwhile divides, and one of the daughter-nuclei 

 afterwards wanders out into the bud. 



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

 somes. B. Equatorial piate 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. 



(3) The asters attain but a slight development, and in some cases 

 appear to be entirely absent (Infusoria). 



(4) The arrangement of the chrpmatin-granules to form chromo- 

 somes appears to be of secondary importance as compared with 

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

 be the fission of the individual granules. 



The basis of our knowledge in this field was laid by Richard 

 Hertwig through his studies on an infusorian, SpirocJiojia {'77), and a 

 rhizopod, ActinospJuErium ('84). In both these forms a typical spin- 



