64 



CELL-DIVISION 



die and equatorial plate are formed inside the nuclear membrane by 

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

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

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

 thouo-h thev did not prove, that these plates arose by the division of a 

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

 alsS described in Actinosphcerinm. and somewhat later by Schewiakoff 

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

 "nucleolus" has since been demonstrated by Keuten m Englena 



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

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

 rounded bv a gmup of chromosomes. B. Division of the " nucleolus " to form an mtra-nuclear 

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



(Ficr 29) and Schaudinn in Amceba. There can therefore be little 

 doifbt 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- 

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

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

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

 Thes? facts indicate, as Richard and Oscar Hertwig have_ con- 

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



