THE BIOLOGY OF THE CELL SURFACE 



are In stages of nuclear division for then the chromosomes 

 are most easily visible. 



The chromosomes also appear as strands of condensed 

 structures lying in the more fluid substance of the nucleus, 

 the so-called karyolymph. When the nucleus is at rest the 

 chromosome-material appears as granules and is designated 

 chromatin. How far this appearance coincides with the 

 real state of the chromosome-material has yet to be deter- 

 mined. Since the chromosomes display regularity in struc- 

 ture and behavior during the process of nuclear division 

 when they are visible and, after fading from view, appear 

 again as bodies of the same number, form and character, it 

 lies close at hand to conceive that they maintain their 

 organization also when they are seen as granules. Theo- 

 retically, for the chromosome-theory of heredity, which will 

 be discussed later, it is also simpler to conceive that the 

 chromosomes always maintain their form and never 

 become disorganized by breaking up Into granules. 



The rapidity with which the nucleus carries out its 

 changes might be considered sufficient for the maintenance 

 of integrity during both the resting and the dividing stage. 

 But the fact that the nucleus remains separated from the 

 cytoplasm may be due to an interposed membrane. 

 Though such a membrane can be demonstrated in many 

 cells, its presence generally in cells has been denied. It is 

 further not at all clear how this membrane arises. As with 

 the presence or absence of the nuclear membrane, so with 

 the linin, a net-work of material In the fixed nucleus: some 

 hold that it is a real structure, others that it is merely an 

 artefact. 



There is present In the nucleus a nucleolus (in some cases 

 there are several nucleoli) which may be a double structure. 

 Its function is not known. Indeed, one can not be sure of 

 its origin or fully follow its history from one cell generation 

 to the next. 



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