73 



the interstices of the linin-frame\vork ; (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 

 m'ore 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." 



FIG. 34. Cyclical vegetative changes in the 

 resting nuclei of TricJwsphccrium 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 prduess 

 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 ; //, the network has become fino- 

 meshed again ; 7, a karyosome is being 

 formed by condensation of the chro- 

 matin at certain points, leading to the 

 condition of A again. After Sclmudinn, 

 X 2,250. 



