THIRTEENTH LECTURE. 



NUCLEAR DIVISION IN PROTOZOA. 



GARY N. CALKINS. 



MITOSIS, or indirect division of the nucleus, with its compli- 

 cated processes, is almost as characteristic of the cell as the 

 nucleus itself. The general agreement of the separate com- 

 ponent parts of the mitotic figure, in most cases down to the 

 finest details of structure, makes it appear as difficult to trace 

 its evolution as to describe the evolution of the cell itself. Yet 

 the cell has evolved from the general to the special, and the 

 mere fact that mitosis is strikingly similar in the most special- 

 ized of animal and plant tissues should not weigh against the 

 view that these mitotic structures have had a history and that 

 some stages in this history may be paralleled by structures to 

 be found in the lowest forms of life. 



All nuclei of higher plants and animals pass, during mitosis, 

 through similar pro-, meta-, and telophases, and a similar divi- 

 sion, or mitotic, figure is formed. But minor variations and 

 differences in what may be homologous structures are seen in 

 the so-called achromatic and chromatic portions of the spindle 

 figure, differences which may be accounted for by the supposi- 

 tion of divergent modification of a common ancestral form. A 

 few of the more important of these variations may be briefly 

 considered before turning to the more generalized structures 

 and processes in Protozoa which show, I believe, a more primi- 

 tive condition. 



Broadly speaking, the variations in mitotic figures may be 

 reduced to three types: (i) Forms with centrosome, central 

 spindle, mantle fibers, and astral rays. (2) Forms with centro- 



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