n] NUCLEOLI; NUCLEAR MEMBRANE 19 



other chromosomes have given rise to the nuclear network, 

 as will be explained in a later chapter. The true nucleolus 

 or plasmosome differs from a karyosome in staining with 

 acid rather than basic dyes, and thus consists of plastin 

 rather than chromatin. It is a very conspicuous object in 

 some nuclei, for example, in those of most eggs ; it usually 

 has a regular rounded outline, and may be vacuolated. It 

 probably serves as a reservoir of nutritive material, for in 

 Protozoa it has been seen to wax and wane according to the 

 good or poor nutrition of the animal. When the nucleus 

 divides the nucleolus usually breaks up and is absorbed, or 

 portions pass irregularly into one or other^of the daughter 

 cells. Although it is usual to separate karyosomes and plas- 

 mosomes rather sharply, on the ground that one consists of 

 chromatin and the other of plastin, it nevertheless may 

 happen that a nucleolus contains both substances, usually 

 in the form of a plasmosome in which are embedded masses 

 or globules of chromatin. As nuclear division approaches 

 the chromatin may increase in amount, and it seems prob- 

 able that the plastin of the nucleolus is converted in these 

 cases into chromatin, which is then transferred to the 

 chromosomes. 



The last chief constituent of the typical nucleus is the 

 membrane, about the nature of which there has been con- 

 siderable divergence of opinion. It is a very definite struc- 

 ture in the resting stage, and is easily seen both in fixed 

 preparations and in the living cell. It usually stains with 

 basic stains, but when exceptionally thick it may also have 

 an outer oxyphil layer. When the nucleus is about to divide 

 the membrane disappears, apparently by a process of solu- 

 tion, and it appears to be formed again from or by the 

 activity of the chromosomes when the daughter nuclei are 

 reconstituted, but opinions differ as to how this process 



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