THE REPRODUCTION OF THE PROTOZOA 



119 



branches, as it were, of the stem which culminates in the Metazoan 

 type. An example of this is seen in the peculiar karyokinesis of 

 Noctiluca (Fig. 65), in which the division is directed by a large 

 " sphere," consisting of a mass of archoplasm containing the cen- 

 trioles. The sphere divides and forms the axis of the karyokinetic 

 figure, of which the nuclear portion is placed asymmetrically to one 

 side. 



In considering this remarkable process of evolution, consisting in 

 the gradual elaboration of a highly complicated mechanism for 

 division ot the nucleus, the question naturally arises, What is the 

 object of a process so elaborate ? Or, if this method of posing the 

 problem offends as being too teleological, we may alter the phrase- 

 ology, and inquire, What is the result of the process ? The answer 

 is perfectly obvious. The result effected by equating karyokinesis 



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A 



FIG. 65. Stages in the nuclear division of Noctiluca miliaris. A, Early stage, 

 the "sphere" (sph.) beginning to divide, the nucleus wrapping round it; 

 B, later stage, the sphere nearly divided, the two poles of the nuclear spindle 

 in section attached to the two daughter-spheres ; C, section across B ; the 

 sphere contains a centriole (c.), to which the chromosomes (chr.) are attached 

 by achromatic fibrils. After Calkins (48). 



in its most perfected forms is an exact halving, both quantitative 

 and qualitative, of the chromatin-substance of the nucleus quanti- 

 tative, by division of each chromatin-granule or chromidiosome, 

 and the partition of the division-products equally between the two 

 daughter-cells ; qualitative also, if we suppose that different cliro- 

 midiosomes may have different properties, and exert their own 

 peculiar influence on the life and activities of the cell ; then, since 

 each daughter-cell contains finally the sister-chromidiosomes of those 

 contained in the sister-cell, the qualities of its chromatin are the 

 exact counterpart of those of the sister-cell and also of the original 

 parent-cell. Hence karyokinesis may be regarded as insuring the 

 transmission to the daughter-cells of the distinctive properties of 

 the parent-cell, unimpaired and unaltered. The whole process indi- 

 cates clearly the immense importance of the chromatin-substance 



