CELL-DIVISION 



is for that on cell-division, we must always reckon with the 

 fact that these cells are not in their natural medium and 

 also that they have escaped the integration under which 

 they live while in the intact organism. Similarly, it is 

 dangerous to interpret the mechanism of cell-division from 

 the study of living cells after these have been dissected out 

 of the organism. Those eggs normally shed and insemi- 

 nated in the sea and free-living protozoa offer the best 

 opportunities for observations on cell-division in the living 

 cell under conditions approximating the normal. Since 

 here we deal with metazoa, I concentrate the discussion 

 on them, restricting myself largely to animal eggs. This 

 I do, not only because this book concerns itself with animal 

 eggs, but also because the most prominent theories on cell- 

 division are based on the study of eggs, especially those of 

 the sea-urchins. Nevertheless, we finally encompass cell- 

 division in all animal forms. 



Obtainable in large number, easy to handle, of convenient 

 size and comparatively simple in structure, eggs of various 

 species of sea-urchins have been most popular cells for the 

 study of cell-division. The cytoplasm of some is trans- 

 parent or nearly so and in them one clearly discerns in the 

 successive stages of nuclear division the behavior of the 

 various constituents of the mitotic complex, including 

 the chromosomes once they appear on the spindle; in others 

 because some cytoplasmic granules are brightly colored, 

 one marks by their shift the ebb and flood of the cytoplasmic 

 tides. The study of the pigmented type supplements that 

 of the transparent. Also, the transparent eggs one can 

 color by means of inert dyes that do no harm to the living 

 cytoplasm, and can in the pigmented ones by innocuous 

 centrifugal force mass the colored granules so that the 

 nuclear phenomena more sharply stand out. But the cells 

 must be normal; when fertilized they should be of the same 

 specific gravity and almost perfect spheres, all showing 



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