THE NUCLEUS 



77 



(Reichenow, 78). As a general rule in the Protozoa, the plastin-sub- 

 stance is found as the matrix of karyosomes, but also as that of 

 other masses of cliromatin, such as the chromosomes of the dividing 

 nucleus (see next chapter). Goldschmidt (41) observed that the 

 formation of generative chromidia in Mastigella (p. 265) was pre- 

 ceded by the extrusion of plastin from the nucleus into the cyto- 

 plasm, to serve as a matrix for the cliromatin which passed out 

 from the nucleus subsequently. In Actinosphcerium, Hertwig has 

 shown that a karyosome or chromatin-nucleotus, present during 

 certain states or phases of nuclear activity, may give off its chro 

 matin-substance into the nuclear framework (reticulum), leaving the 

 plastin- matrix as a body which is then seen to consist of a reticular 

 framework similar in structure to the achromatinic reticulum of the 

 nuclear framework, but distinguished from it by smaller meshes 

 (alveoli) enclosed by thicker walls, as well as by its different staining 

 properties. Certain phases of the development of Actinosphcerium 

 are further characterized by the formation in the nucleus of 

 numerous small plastin-nucleoli, each consisting of a single vesicle 

 (alveolus) of plastin containing nuclear sap. 



Thus, a nucleus in its full complication of structure, and apart 

 from the centrosomic elements, to be discussed presently, consists 

 of the following parts : (1) An achromatinic framework or nuclear 

 reticulum ; (2) a true membrane, formed from the achromatinic 

 framework, and separating the nuclear contents from the surround- 

 ing cytoplasm ; (3) nuclear sap, pervading the entire nuclear cavity ; 

 (4) plastin, in the form of one or more bodies or masses which may 

 consist either of pure plastin (nucleolij or of plastin impregnated 

 with chromatin (karyosomes) ; and (5) the cliromatin, which may 

 be present either in the form of granules lodged at the nodal points 

 of the reticulum, and scattered evenly or unevenly throughout the 

 nuclear framework, or may be concentrated in a karyosome, or 

 may combine both these two modes of distribution in various ways. 

 Achromatinic framework and nuclear sap may be considered as a 

 part of the general body-protoplasm, enclosed within the nuclear 

 space, and set apart from the cytoplasm as a special nucleoplasm ; 

 plastin, on the other hand, is probably to be regarded as a product 

 derived from the chromatiii itself, either as a secretion or as a 

 modification of its substance, to form a cement-like material or 

 matrix in which true chromatin is carried. The two primary con- 

 stituents of a nucleus are chromatin and protoplasmic framework. 



Nuclei, whatever their structure, are, as a general rule, bodies of 

 spherical or ovoid form ; but in some cases, especially amongst 

 Infusoria, the nuclei exhibit very varied forms in different species. 

 The nucleus may then be sausage-shaped, or in the form of a horse- 

 shoe, or resemble a string of beads (" moiiiliforni "), or be branched 



