66 THE PROTOZOA 



throughout the cell, or may be aggregated in certain regions of 

 the body to form ' chromidial masses ' or ' chromidial nets." 

 It is even found that in some species a true nucleus may be absent 

 temporarily during some phases of the life-cycle, all the chromatin 

 being then in the form of chromidia, from which nuclei arise by a 

 process of condensation and organization of the chromatin in com- 

 bination with achromatinic elements. Such a condition may be 

 regarded as a temporary reversion to a more archaic and ancestral 

 condition, since, as has been pointed out already (Chapter I.), the 

 Protista of the lower or bacterial grade of organization do not 

 possess, speaking generally, a true nucleus, but only scattered 

 grains of chromatin. Hence the chromidial condition of the 

 chromatin may be ranked as an earlier and more primitive state, 

 from which the strictly cellular grade of organization has been 

 evolved by concentration of some or all of the chromatin to form a 

 nucleus. In the tissue-cells of Metazoa, as a general rule, and in 

 many Protozoa, the chromatin is concentrated entirely in the nucleus 

 or nuclei, and chromidia do not occur. 



Whatever view be taken as to the primitive or secondary nature 

 of the chromidial condition (a question upon which individual 

 opinions may differ considerably), the following facts can be stated 

 definitely with regard to the chromidia. In some cases the chromidia 

 can be observed to arise as extrusions of chromatin from the nucleus, 

 which either casts off a certain amount of chromatin into the cyto- 

 plasm, while preserving its individuality, or may undergo complete 

 fragmentation, becoming resolved entirely into chromidia, and 

 ceasing to exist as a definite nucleus. In other cases, chromidia 

 arise from pre-existing chromidia, by growth and multiplication 

 of the chromidiosomes, thus keeping up a chromidial mass or stock 

 which is propagated from cell to cell through many generations, 

 independently of the nuclei present in addition to them in the cell. 



The chromidial mass itself may vary considerably in structure 

 in different cases or at different seasons ; the chromidiosomes may 

 be arranged in clumps, strands, or trabeculae, on a protoplasmic 

 framework, and the mass is often vacuolated and contains substances 

 other than chromatin. In Difflugia, Zuelzer (85) has shown that 

 in the autumn the chromidial mass assumes a vacuolated or alveolar 

 structure, and in each alveolus grains are formed of a carbohydrate 

 substance allied to glycogen, which functions as reserve food- 

 material for the organism during the reproductive processes initiated 

 at that season. 



On the other hand, as chromidia arise from nuclei, so nuclei may 

 arise from chromidia. In many Protozoa, as, for example, Arcella 

 (Fig. 32), the formation of so-called "secondary' nuclei (which, 

 however, do not differ from other nuclei except in their mode of 



