THE CELL. 35 



plasm. 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 used only in 

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

 pounds, though they are distinguished from each other by staining 

 affinities which are based on chemical differences. The chromatin 

 has been further resolved into a ground-substance, giving form to the 

 reticulum of the nucleus and called linin, and to granules of chromatin 

 (the term being used in a more restricted sense), and this chromatin has 

 in turn been subdivided according to staining characteristics, into oxy- 

 chromatin (or lanthanin) and basichromatin, embedded in the linin 

 in the form of minute granules. The achromatin has also been 

 observed to contain granules, not stained with the usual nuclear dyes, 

 which have been called oedematin granules. The behavior of the 

 nucleoli toward dyes is somewhat different from that of the chromo- 

 plasm, 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- 



