60 PROTOZOAN NUCLEUS 



Between these two types of nuclei many intermediate forms are found, 

 and individual variations are of common occurrence. These variations 

 may affect the nuclear membrane, which may be exceedingly fine in some 

 forms and comparatively thick and dense in others. The arrangement 

 of the chromatin upon the membrane may be in the form of uniformly 

 distributed fine granules, or there may be coarse granules more irregularly 

 distributed, or most of the chromatin may be aggregated into a semi- 

 lunar mass planted on one side of the membrane. 



The linin netw^ork itself may be in the form of a uniform mesh, or it 

 may consist of radially arranged strands. The meshes of the network 

 may contain granules other than chromatin or globules of an undetermined 

 nature. The minute structure of the nuclei is of considerable importance 

 in the differentiation of species. 



Though it is not possible to draw a hard-and-fast line between those 

 nuclei which possess karyosomes and those which do not, there never- 

 theless exist certain nuclei in which there appears to be no tendency 

 towards the formation of a central structure. Amongst the gregarines, 

 for instance, certain individuals of a particular species may show a single 

 deeply staining body in the nucleus, or more than one, while in some 

 Protozoa there are a series of deeply staining bodies upon the nuclear 

 membrane, while the interior of the nucleus is occupied by a uniform 

 meshwork of fibrils. It appears impossible to speak of several bodies 

 in the nucleus as karyosomes, a term which is undoubtedly used by the 

 vast majority of zoologists, for the single more or less centrally placed 

 structure described above. Nuclei of the type which has no definite 

 karyosome may, however, contain a body which may or may not be 

 central in position, and which is regarded as devoid of chromatin, owing 

 to the fact that it does not stain intensely with chromatin stains. Such 

 a structure occurs, according to Metcalf (1909), in the nuclei of species 

 of Opalina, in which a deeply staining karyosome is not present. It 

 resembles. the nucleolus which is commonly found in the nuclei of the 

 cells of higher animals. Bodies of this type have also been described by 

 Reichenow (1921) in the nuclei of various stages of development of the 

 hsemogregarines of the genus Karyolysus. They are also present in the 

 nuclei of Hepatozoon balfouri, and, as in the case of Karyolysus, they divide 

 during nuclear division (Fig. 35). They are commonly present in the 

 nuclei of coccidia. They take no part in the formation of the spindle or 

 the chromosomes. 



Another type of nucleus which has to be mentioned is the macro- 

 nucleus of the Euciliata. It has already been explained that these 

 Protozoa typically possess two nuclei — the micronucleus and the macro- 

 nucleus. The former is usually of the type which contains a central 



