738 CALKINS. [Vol. XV. 



the ends of the chromosomes with them. As the central- 

 spindle becomes longer, the chromosomes are more and more 

 separated, until finally the distal ends are separated and chro- 

 mosome division is completed. 



R. Hertwig ('95) has already pointed out that this view of 

 the mechanics of mitosis is perfectly consistent with the facts 

 of nuclear division in the Infusoria. No mechanical hypothe- 

 sis, however, can fully explain the different phenomena in- 

 volved in the mitosis of Noctihica. The action of the central- 

 spindle cannot explain the first elongation of the nucleus in 

 the primary axis, nor the later ring-form. Neither does it 

 offer any explanation of the forces which cause the in-sinking 

 of the central-spindle into the position of the secondary nuclear 

 axis. The ultimate cause of these phenomena remains unex- 

 plained, and there seems to be little doubt that something 

 deeper than mere mechanical force is necessary to explain 

 mitotic activity. This is strikingly confirmed by Juel's obser- 

 vation on the isolated chromosome of Hciiicrocallis fiiha and 

 by Boveri's recent ('97) experiments on sea-urchin eggs in which 

 he found that blastomeres are incapable of dividing when 

 chromatin is absent. 



III. THE NUCLEAR RELATIONS OF NOCTILUCA TO METAZOA 

 AND PROTOZOA. 



A. Relations to Metazoa. 



The similarity of the mitotic-figures of Noctiluca to those of 

 the Metazoa has already been indicated by Ishikawa. The 

 comparison can now be carried still further. They agree in 

 the following points : (i) the central-spindle fibers end in 

 spheres which contain centrosomes ; (2) the central-spindle 

 occupies a position in the center of the nuclear plate ; (3) the 

 chromosomes lie freely around it without an intervening nuclear 

 membrane ; (4) the central-spindle fibers are not connected 

 with the chromosomes ; (5) mantle-fibers connect the chromo- 

 somes with the centrosomes ; (6) the chromosomes in Noctiluca, 

 like those of the Metazoa, are composed of granules which are 

 at first separate, then unite to form a segmented spireme, 



