CHAPTER VI 



THE ORGANIZATION OP THE PROTOZOA (Continued) THE 



NUCLEAR APPARATUS CHRO MATIN, NUCLEUS, 

 CHROMIDIA, CENTROSOMES, AND BLEPHAROPLASTS 



OF all the parts or organs of the cell-body, there is none of greater 

 importance for the life and activities of the organism than the 

 so-called nucleus, a term which, understood literally, means simply 

 a kernel or central portion of the body, and conveys no idea of the 

 true nature of the structure in question or of its significance for the 

 life of the organism. 



The cell-nucleus, in all its various modifications, of form and 

 structure, is essentially and primarily a collection of grains and 

 particles of a peculiar substance which has received the name 

 chromatin, on account of its characteristic tendency to combine 

 with certain colouring matters and dyes. A nucleus may consist, 

 perhaps, in some cases of little more than a single mass of chromatin, 

 or of several such masses clumped together. In most cases, how- 

 ever, the chromatin is combined with other substances which may 

 be termed comprehensively achromatin, and which are built up with 

 the chromatin in such a way- as to produce a complicated nuclear 

 structure, as will be described in detail presently. 



The chromatin-substance is not necessarily, however, concen- 

 trated entirely in the nucleus in all cases. In many Protozoa, 

 especially amongst the Sarcodina, as, for example, Arcella (Fig. 32), 

 Difflugia, and many other genera, the cell-body contains, in addi- 

 tion to one or more nuclei, extranuclear granules of chromatin, 

 termed chromi'dia,* which may be scattered in the cytoplasm 



* The term " chromidia," in the German form " Chromidien," was coined by 

 Hertwig (66) to denote the extranuclear grains of chromatin, and the whole mass 

 of them in the cell-body was spoken of as a " Chromidialnetz." Subsequent 

 authors, however, have used the word in its singular form, " chromidium," in a 

 collective sense, to denote the entire mass of chrcmidia present in a cellular organ- 

 ism, and not, as might have been expected, to mean the individual grains or 

 particles of chromatin which constitute the chromidial mass. In order to avoid 

 confusion, it is proposed in this work to use the term chromidiosome to denote the 

 smallest chromatin-particles oi which the chromidial mass is made up, and which 

 grow and multiply by division like other elementary living bodies. It is clear, 

 however, that the chromidiosomes of which the chromidial mass scattered in the 

 cytoplasm is built up are in no way different in kind from the minutest granules 

 of chromatin contained in the nucleus. The term "chromidiosome" must there- 

 fore be applied to the ultimate, individual grain or particle of chromatin, alike 

 whether it be lodged inside or outside a nucleus. 



65 f> 



