STRUCTURE OF NUCLEUS 59 



though it may give useful information as to the type of cell or parasite 

 present, is completely misleading when it comes to a consideration of the 

 minute nuclear structure. Descriptions of the characters of nuclei which 

 are based on preparations of this kind are not only worthless but misleading. 



From the above description it will be realized that the nuclei of Protozoa 

 may be roughly divided into two classes: those in which there is a central 

 karyosome, and those in which no such karyosome is present. Compared 

 with the size of the nucleus, the karyosome may be a relatively small 

 structure, or it may occupy a large part of its bulk. As a type of nucleus 

 with small karyosome, that of Entamoeba histolytica will serve as an illus- 

 tration (Fig. 95). There is a definite nuclear membrane, on the inner 

 surface of which practically all the chromatin is arranged in the form of 

 small granules. At the centre of the nucleus is a small granule, the 

 karyosome, which presumably consists of plastin material and possibly 

 some chromatin. Surrounding the karyosome is a clear area, the limits of 

 which form a sphere (or ring in optical section) of fine granules. These do 

 not contain chromatin, but represent the inner limit of the linin network 

 which connects the sphere with the nuclear membrane. The linin network 

 appears to be free from chromatin. Dobell (1919), who has studied the 

 nuclear division in this amoeba, could obtain no evidence of the existence 

 of a centriole in the karyosome, though such a structure has been described 

 by Hartmann (1908-1913). During division of the nuclei within the cysts 

 the writer has seen forms which suggest the presence of a central granule 

 which divides (Fig. 57). 



A type of nucleus in which a definite and relatively large karyosome is 

 present is of frequent occurrence. It is seen typically in trypanosomes, 

 many free-living amoebae, and other Protozoa (Figs. 48, 89, 224). These 

 karyosomes are comparatively large structures which are connected with 

 the nuclear membrane by the linin network. It is possible, though by 

 no means certain, that some of the chromatin of the nucleus may be con- 

 centrated in the karyosome, which stains intensely with certain nuclear 

 stains. The nuclear membrane and the linin network may have com- 

 paratively little chromatin, which in small nuclei, such as those of trypano- 

 somes, is difficult to detect. In many cases nuclei of this type are 

 described as possessing centrioles within the karyosomes. The large 

 karyosome may appear perfectly uniform and homogeneous, or it may 

 show indications in stained specimens of being composed of a varying 

 number of deeply staining bodies embedded in a more faintly staining 

 l)lastin matrix. The karyosome is often spherical in form, but it may be 

 irregular in shape. In some cases on the surface of the karyosome there 

 occur deeply staining granules, which may be chromatin, while the central 

 part consists of })lastin. Sometimes one or more vacuoles are present. 



