NUCLEAR DIVISION IN PROTOZOA. 223 



cytoplasmic body (archoplasm), or as a kinetic substance (kino- 

 plasm), it may undergo further differentiation leading to the 

 complicated mitotic figures of higher animals and plants. In 

 all cases, however, the nuclear membrane disappears during 

 mitosis (save perhaps in Paminaba and diatoms), and the origi- 

 nal or primitive condition is thus repeated. By the disappear- 

 ance of the membrane, the substance of the sphere again comes 

 into direct contact with the chromatin. 



Among the differentiations which the extranuclear sphere 

 undergoes is the formation of spindle fibers, of centrosomes 

 (or centrioles of Boveri and Hertwig), and of astral or cyto- 

 plasmic rays. The spindle fibers in their simplest form arise 

 as a fibrous arrangement of the sphere or archoplasm. In 

 Parama'ba there is no indication of fibers ; the mass of the 

 sphere merely pulls out until the ever-thinning connecting 

 strand is broken. In Noctiluca the connecting strand becomes 

 distinctly fibrillated, the fibers consisting of a linear arrange- 

 ment of the microsomes which compose the sphere. In this 

 condition the strand may be called the central spindle, for its 

 subsequent position in the mitotic figure shows that it has the 

 same relations with the chromatin as the central spindle of the 

 Metazoa, where, according to numerous observers, it arises in 

 a similar way from the material about the centrosome (archo- 

 plasm according to Boveri, from substance of the centrodesmus 

 according to Heidenhain). In many cases, however, the central 

 spindle of the Metazoa is only a transient structure, the fibers 

 of which it is composed sooner or later breaking across, so 

 that it does not penetrate the nucleus as a spindle (many Q.gg 

 cells). 



In plant cells, on the other hand, quite a different process 

 seems to have taken place. Instead of a growing concentra- 

 tion of the sphere into a definite cell structure, there has been 

 apparently an ever-increasing diffusion of its substance, until, 

 as Strasburger states it, the cytoplasm is everywhere penetrated 

 by kinoplasm, which, prior i;o nuclear division, comes together 

 in lines to form the extranuclear spindle. 



In those Protozoa where the sphere has become an intra- 

 nuclear structure a similar history may be postulated. In 



