Page:Popular Science Monthly Volume 85.djvu/126

122 divide unequally and differentially, and this is probably a prime factor in development.

On the other hand, the differential division of the cytoplasm is a regular and characteristic feature of ontogeny—indeed the segregation and isolation of different kinds of cytoplasm in different cells is the most important function of cell division in development. Thus we find in the division apparatus of the cell a mechanism for the preservation

 ''Ascaris. A and B,'' second cleavage division showing that the chromosomes remain entire in the lower cell, which is in the line of descent of the sex cells ("germ track"), but that they throw off their ends and break up into small granules in the upper cells, which become somatic cells. C, 4-cell stage, the nuclei in the upper somatic cells being small and the ends of the chromosomes remaining as chromatic masses in the cell body outside of the nuclei, while the nuclei in the lower cells are much larger and contain all the chromatin. D, third nuclear division, showing the somatic differentiation of the chromosomes in all the cells except the lower right one, which alone is in the germ track and will ultimately give rise to sex cells. (After Boveri.)

in unaltered form of the species plasm, idioplasm or germ-plasm of the nucleus, and for the progressive differentiation of the personal plasm or somatoplasm of the cell body.