THE FUNDAMENTAL ORGANIZATION 



59 



despite the fact that very little definite information is at hand con- 

 cerning it. In pure form it is the nucleolus of tissue cells and stains 

 intensely with acid dyes. Such nucleoli are rare in Protozoa, but 

 the combination of plastin with chromatin in some degree is char- 

 acteristic of Protozoa, and the staining reaction with basic or acidic 

 dyes varies with the preponderance of one or the other. 



The ground-substance of the nucleus or karyolymph (Lundegardh) 

 is difficult to define, a difficulty which Belar (1926) recognizes by 

 the statement: ". . . at best it can be defined as that part of 

 the nuclear space which is neither chromatin nor plastin" (loc. cit., 

 p. 242). From this negative definition and from the fact that it 

 cannot be demonstrated by specific staining reactions or character- 

 ized by definite structures, it might seem that karyolymph is a 

 negligible part of the nuclear make-up. Such a conclusion, how- 

 ever, would be a mistake for some of the most important structures 

 of the active nucleus take their origin from this ground substance 

 (see pp. 88, 200). 



Fig. 28. — Vahlkampfia Umax; chromatin forming the nuclear membrane and giving 

 rise to chromidia. (After Calkins.) 



Membrane. Like other constituent parts of the protozoon nuclei, 

 the membranes are highly variable, sometimes presenting in optical 

 section only one contour on the outer side (e. g., Actinosphaerium) ; 

 sometimes showing contours both outside and inside (Amoeba pro- 

 teus) . In the former case the inner zone adjacent to the membrane 

 shows a decreasing density inwards, until the linin merges insen- 

 sibly into the intranuclear reticulum. In free-nuclei formation, 

 antecedent to gamete formation described above, the nuclear mem- 

 branes are probably formed from the cytoplasmic reticulum in 

 which the chromidiosomes are lying. Chromomeres also take part 

 in the formation of nuclear membranes in some cases, e. g., in 

 Vahlkampfia Umax, where the linin membrane is too delicate to be 

 seen, although the definite limitation of the chromomeres indicates 

 its presence (Fig. 28). 



One peculiarity of the nuclear membranes of Protozoa which dis- 

 tinguishes them from nuclear membranes of tissue nuclei, is that in 

 the majority of cases they remain intact during all phases of cellular 

 activity and only rarely disappear, or disappear in part only, during 

 division processes of the cell. (For description of chromatin, mem- 

 branes, etc., during division, see p. 209.) 



