NUCLEAR DIVISION IN PROTOZOA. 227 



fibers have nothing to do with chromosomes. The intranu- 

 clear fibers of forms with pole plates thus correspond to 

 mantle fibers of the Metazoa and Noctiluca, and the evidence 

 is strong that they, like the central spindle fibers, are formed 

 from the substance of the intranuclear sphere. The origin 

 of the extranuclear cytoplasmic masses from the intranuclear 

 sphere {ActinospJicej'umi) lends support to the theory, first 

 advanced by Hertwig, that the centrosome arises by emergence 

 from the nucleus, while it also explains the origin of mantle 

 fibers from linin substance of the nucleus as described by 

 numerous observers upon different forms of Metazoa. 



Summary and Conclusions. 



The component parts of the mitotic spindle in Metazoa may 

 be thus traced back to the generalized condition found in many 

 Protozoa. Although many points are obscure and need verifi- 

 cation, one fact, at least, appears well supported, viz., the 

 so-called achromatic and chromatic portions of the mitotic 

 figure were originally more or less independent bodies which 

 came together only during mitosis. The view advanced by 

 Lauterborn, that the original form of the kinetic center was in 

 some binucleated ancestor such as Amceba bmucleata, meets 

 with many obstacles. It must be assumed, according to this 

 theory, that one nucleus not only changed its form, but also 

 became completely changed in function. Again, it must be 

 assumed that differentiation of this "kinetic" nucleus must 

 have resulted in the well-differentiated cytoplasmic body 

 (sphere) of Tetramitus or Paramosba as well as the intranu- 

 clear centers of many flagellates, forms which are probably as 

 low in the scale of living things as Amccba. It seems hardly 

 necessary to regard this body, in its original condition, as a 

 nucleus which later becomes differentiated into a well-defined 

 kinetic substance with degeneration of the original nuclear 

 contents. It seems much more simple, and, in the light of 

 facts, much more probable, that the kinetic substance was as 

 definite a portion of the original primitive cell as the nucleus 

 itself, although formed possibly from metamorphosed chroma- 



