I NUCLEAR CONSTITUENTS 5 



The structure of the nucleus varies profoundly according as to whether 

 it is going through the processes connected with nuclear division, or is 

 in the phase between two division periods. In the latter condition, 

 which we will consider first, the nucleus is said to be at " rest." It must, 

 however, be understood that this word implies merely that the nucleus is 

 at rest from division. The " resting nucleus " is doubtless in the phase 

 of its greatest physiological activity. 



A typical Metazoan resting nucleus consists of the following parts : 

 (i) chromatin ; (2) linin ; (3) nuclear sap or karyolymph ; (4) karyosome ; 



(5) plasmosome — the two last mentioned both being known as nucleoli ; 

 they may or may not be present, and if present they may be multiple ; 



(6) miclear membrane. 



The arrangement of these constituents varies greatly, the commonest 

 disposition being such that the linin is a faintly staining substance form- 

 ing a spongework stretching throughout the nucleus and containing 

 embedded in it the chromatin — a substance which colours intensely 

 with most stains. The meshes of the Unin-chromatin reticulum — or 

 rather spongework — so formed are filled with the fluid karyolymph. 

 Karyosomes are larger aggregations of chromatin, but the term is incap- 

 able of exact definition. It is in practice restricted to comparatively 

 large chromatin masses occurring in nuclei in which the rest of the 

 chromatin (if any) is finely distributed. Nuclei of a coarser structure 

 may contain equally large aggregations of chromatin at the nodes of the 

 reticulum, though these are not generally called karyosomes. 



Plasmosomes are composed of a substance called plastin, which is 

 different in nature from any of the above-mentioned substances. Many 

 nucleoli, however, are incapable of classification as karyosomes or 

 plasmosomes, since they partake of the nature of both, consisting of 

 plastin impregnated with chromatin. Examples of this kind of nucleolus 

 (sometimes called an amphinucleohis) are found in the " karyosomes " 

 of many Protista, and probably in the nucleoli of certain Metazoan 

 oocytes. True plasmosomes disappear before nuclear division and are 

 reformed in the young daughter nucleus. They are doubtless of a 

 metaplastic nature. 



Since it is impossible to appreciate the nature of the various consti- 

 tuents of the nucleus without knowledge of their behaviour during 

 nuclear division, further discussion of them will be postponed till after 

 we have studied this phase in the life of the nucleus. 



A nucleus divides to form two nuclei in one of two ways, the one 

 being known as indirect division, mitosis or karyokinesis, and the other 

 as direct division or amitosis. The overwhelming majority of nuclear 

 divisions among the Metazoa and Mctaphyta are of the first or mitotic 

 type, which we will now proceed to consider. 



