CHAPTER I 

 THE RESTING NUCLEUS 



THE term ' resting nucleus ' is unfortunate, since it 

 seems to imply that the metabolic activities of 

 the nucleus are reduced to a minimum when it is not 

 dividing — a view for which there is no evidence. The 

 alternative term ' metabolic nucleus ' is equally un- 

 fortunate in that it suggests that dividing nuclei are 

 physiologically inactive. On the whole it seems best 

 to retain the established, although misleading, term. 



The resting nucleus is, then, one which is not divid- 

 ing. Usually it remains optically unaltered for long 

 periods — it is not obviously changing either its shape 

 or its appearance. This is, however, not always so ; 

 many resting nuclei increase steadily in size (either by 

 uptake of water or by actual ' growth ') and may alter 

 their shape or appearance in various ways without 

 dividing. 



The chief structural parts of the resting nucleus 

 are the nuclear membraney the nuclear sap and the 

 chromosomes ; these three constituents are always 

 present — in addition bodies known as nucleoli may 

 also be present. It has been suggested ^^ that the 

 latter are to be regarded as portions of cjrtoplasmic 

 material included in the nucleus, but this is not so ; 

 true portions of the cytoplasm may become accident- 

 ally included in the nucleus, and are then seen to be 

 quite different in appearance from either nucleoli or 

 nuclear sap.^'^ 



The nuclear membrane has been shown by micro- 

 dissection to be a definite structure with physical 

 properties. If a fine needle is gently pushed against 



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