SPECIAL MORPHOLOGY OF THE PROTOZOAN NUCLEUS 249 



which characterize the typical nucleus of Metazoa are rarely present 

 here in one and the same nucleus. The nuclear membrane in some 

 cases is absent, in other cases well defined and persistent throughout 

 active as well as resting stages. The linin reticulum, with its inclosed 

 chromatin granules, is frequently absent, while the ground substance 

 of the nuclei, or the karyolymph, although it has not been critically 

 examined, is undoubtedly present in the majority of cases. Nucleoli 

 or plasmosomes have apparently been found in but one instance 

 (Actinospfuzrium, Hertwig, '98). The nuclei of Protozoa are, as a 

 rule, further distinguished from those of the Metazoa by the presence 

 of another intra-nuclear body, which apparently corresponds to the 

 sphere and centrosome of the Metazoa, and, as in the latter, plays a 

 prominent part in division. Boveri ('01) has recently proposed the 

 name Centronucleus for nuclei with these division centres. We can 

 distinguish, then, in the various nuclei of Protozoa, (i)a nuclear mem- 

 brane; (2) a nuclear reticulum or linin network; (3) nucleoli, or 

 plasmosomes (in rare cases); (4) chromatin, occasionally in the form 

 of granules strung upon a linin reticulum ; (5) spheres or division- 

 centres analogous to the extra-nuclear kinetic centres in Metazoa, but 

 for the most part intra-nuclear in the Protozoa. 



A. THE NUCLEAR MEMBRANE 



The usual definition of a nucleus includes the membrane as an 

 integral part, sharply separating cytoplasmic and nuclear structures. 

 In a few Protozoa there is no indication of a membrane, and the 

 usual constituents of the nucleus are distributed throughout the cell. 

 In others, notably in the majority of the Sporozoa, the nuclear 

 membrane, as in most Metazoa, disappears during mitosis. In 

 all other cases, however, the nuclear membrane persists, in part at 

 least, throughout active as well as resting phases. It frequently ap- 

 pears as a very faint structure scarcely to be distinguished from the 

 cytoplasmic reticulum (many Flagellidia), although increasing grades 

 of density may be obtained, which culminate in thick resisting 

 membranes like those of Noctiluca, Amoeba proteus, and Ciliata. 

 The membrane of an isolated macronucleus of the Ciliata becomes 

 separated from the nuclear contents and ultimately dissolves in the 

 water (Biitschli). The great rigidity of this membrane and its per- 

 sistence during division of the nucleus led to the view that it is not 

 identical with a nuclear membrane of a higher cell (Biitschli, '76, '83; 

 Hertwig, '76), and some observers even regarded it as highly differ- 

 entiated protoplasm in the form of chitin (Stein, '54, Opercularia), or 

 cellulose (Brandt, '82, Amceba}. 



In the Cystoflagellate Noctiluca miliaris, one part of the membrane 



