6 4 



CELL-DIVISION 



die and equatorial plate are formed inside the nuclear membrane by 

 a transformation of the nuclear substance. In SpirocJiona (Fig. 27, 

 A-C) a hemispherical "end-plate" or "pole-plate" is situated at 

 either pole of the spindle, and Hertwig's observations indicated, 

 though they did not prove, that these plates arose by the division of a 

 large "nucleolus." Pole-plates of a somewhat different form were 

 also described in Actinosph&rium, and somewhat later by Schewiakoff 

 ('88) in EuglypJia (Fig. 28). Their origin through division of the 

 " nucleolus " has since been demonstrated by Keuten in Euglena 



Fig. 29. Mitosis in the Flagellate Euglena. [KEUTEN.] 



A. Preparing for division ; the nucleus contains a " nucleolus " or nucleolo-centrosome sur- 

 rounded by a group of chromosomes. B. Division of the " nucleolus " to form an intra-nuclear 

 spindle. C. Later stage. D. The nuclear division completed. 



(Fig. 29) and Schaudinn in Amoeba. There can therefore be little 

 doubt that the " nucleolus " in these forms represents an intra- 

 nuclear centrosome, and that the pole-plates are the daughter-centro- 

 somes or attraction-spheres. Richard Hertwig's latest work ('95) 

 indicates that a similar process occurs in the micronuclei of Para- 

 mcecium, which at first contain a large " nucleolus " and afterwards 

 a conspicuous pole-plate at either end of the spindle (Fig. 27, D-H\ 

 The origin of the pole-plates was not, however, positively determined. 

 These facts indicate, as Richard and Oscar Hertwig have con- 

 cluded, that the centrosome, in its most primitive form, is an intra- 



