THE NUCLEUS 



73 



the interstices of the linin-framework ; (3) plastin, a substance 

 which has staining reactions different to those of chromatin, and 

 which occurs in lumps or masses forming the ground-substance of 

 the nucleoli or karyosomes presently to be described. The whole 

 nucleus is commonly enclosed in a membrane, but this structure is 

 probably formed in different ways in different cases, and may be 

 absent. In addition to these 

 various constituents, there are 

 commonly present also in con- 

 nection with nuclei bodies of 

 kinetic nature. Such are the 

 centrosomes or centrioles, which 

 appear to control, or at least 

 to act as centres for, the move- 

 ments which the various parts 

 of the nucleus perform during 

 the process of reproduction by 

 division. 



The structure and appear- 

 ance of nuclei depend chiefly 

 on the manner in which the 

 chromatin is distributed. Two 

 principal types of structure may 

 be distinguished : in the first 

 the chromatin is concentrated 

 into a single mass or grain, or, if 

 other grains are present in the 

 nucleus, they are smaller and 

 relatively insignificant in size ; 

 in the second a number of 

 grains are present which are 

 more or less equal in size. In 

 the condition with a single, 

 or one greatly preponderating, 

 mass of chromatin, the nuclear 

 space is not as a rule filled by 

 it, but presents the appear- 

 ance of a vesicle containing 

 the chromatin-mass at or near its centre ; consequently such nuclei 

 are commonly termed " vesicular " in type, and the chromatinic 

 mass may be termed generally, and without further determination 

 of its precise nature, an endosome (" Binnenkorper "). When, on 

 the other hand, the chromatin is in the form of numerous grains, 

 they are generally distributed more or less evenly throughout the 

 nuclear cavity ; such nuclei are termed " granular." 



resting nuclei of Trichosphcerium sieboldi. 



A, Stage with finely-meshed chromatic 

 network and large karyosome (see p. 76) ; 



B, the meshes of the network widening, 

 the karyosome budding off blocks of 

 chromatin into it ; C, the same process 

 carried farther ; D, coarse network con- 

 taining scanty chromatin at the nodes, 

 karyosome wanting ; E to G, the chro- 

 matin increases greatly in quantity, 

 covering the linin-framework in G the 

 meshes of the network are becoming 

 finer ; H, the network has become fine- 

 meshed again ; /, a karyosome is being 

 formed by condensation of the chro- 

 matin at certain points, leading to the 

 condition of A again. After Schaudinn, 



X 2,250. 



