34 NORMAL HISTOLOGY. 



Owing to their affinity for certain coloring matters, the substances 

 composing the nuclear filaments are called chromatin, or chromo- 

 plasrn. The hyaline substances making up the rest of the nucleus 

 do not receive those coloring matters, and for this reason and in 

 this situation are called achromatin. These terms are only used in 

 a morphological sense and do not specify any definite chemical com- 

 pounds. The behavior of the nucleoli toward dyes is somewhat 

 different from that of the chromoplasm, which leads to the inference 

 that they are of a different chemical nature. 



Except during cell-division, the nucleus usually lies quiescent 

 within the cytoplasm, but some observers have seen it execute ap- 

 parently spontaneous movements, and it is evidently possible for its 

 position in the cell to vary from time to time. 



In marked contrast to this apparently dormant state, as far as 

 visible alterations of structure are concerned, is the role played by 

 the nucleus during the reproduction of the cell. 



There are two modes of cell-division, the " indirect " and the 

 " direct," but they are by no means equivalent to each other. The 

 former, also termed karyokinesis because of the active changes in 

 the nucleus, appears to be the only truly reproductive process. 

 Direct cell-division results in the formation of new cells, but they 

 seem to lack that perfection of organization which would be required 

 for the complete and indefinite transmission of all the characters of 

 the parent cells. 



Before entering into a description of karyokinesis, a few words 

 must be said concerning the centrosome. This is an extremely min- 

 ute granule which is usually situated in the cytoplasm not far from 

 the nucleus. It is often surrounded by a thin zone of hyaloplasm 

 which facilitates its recognition among the fibres and nodal points 

 of union of the spongioplasm. The fibres of the latter are also fre- 

 quently arranged in a radial manner for a short distance around the 

 centrosome. But in many instances it is extremely difficult to dis- 

 tinguish the centrosome, and its constant presence in cells is largely 

 a matter of inference. Sometimes the centrosome is double, the 

 two granules lying close to each other and often being surrounded 

 by a common clear zone of hyaloplasm. 



The first step in the process of cell-division by the indirect method, 

 or karyokinesis, is a division of the centrosome into halves (Fig. 

 15), which separate and pass to opposite points in the cytoplasm. 

 These points are called the poles of the cell, and when the new cen- 



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