THE CELL AND CELL DIVISION 



71 



nucleolus or karyosome (e.g., Asterias), and in preparation for mitosis 

 some granules pass out of the karyosome and form into distinct chro- 

 mosomes while the greater part of the karyosome dissolves (Fig. 35); 

 it is difficult to understand how the chromosomes could have preserved 

 their integrity through such a history. In amitosis no chromosomes 

 are visible yet the nuclei of cells thus formed seem to function normally 

 and such cells are capable of typical differentiations and in a few in- 

 stances are said, with some question, however, to be subsequently 

 capable of division by mitosis, and of normal chromosome formation. 



x_V\ ,.', 



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FIG. 35. Primary oocyte of the star-fish, Asterias forbesii, at beginning of 

 division. From Dahlgren and Kepner (Jordan). Chromatin leaving the " chro- 

 matin reservoir" or chromatin nucleolus, and being added to the chromosomes. 



Such an interruption of a series of mitotic divisions by a period of 

 amitotic division would seem to exclude the possibility of both genetic 

 relation and specificity of the chromosomes. In those forms where 

 chromosomes are not normally formed, the chromatin granules are the 

 units in nuclear division; and even when these are formed into chromo- 

 somes the essential step in nuclear division is the fission of these granules, 

 which thus seem to be the real units of the chromatic substance. Yet 

 one could hardly maintain the genetic continuity of these granules upon 

 other than logical grounds, and to many there seems no stopping place 



