NUCLEI AND KINETIC ELEMENTS 



67 



plasmic substances under the action of the kiHin^ fluids and are 

 ahvays to be interpreted as artefacts. The chromidiosomes are 

 suspended in, and held in place by, the linin reticuhnn, which in 

 some cases is extremely delicate and difficult to see, while the inter- 

 alveolar spaces are filled with fluid enchylema {e. g., Arcella vukjaris). 

 In other cases, owing to the suspension of very fine chromomeres, the 

 outlines of the intranuclear alveoli are characteristically distinct 

 {e. g., Acineta grandis). In most massive nuclei, however, the 

 comparati\ely large chromidiosomes distort the alveolar walls, 

 more or less completely obliterating the reticulum. 



{c) 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., Adino- 

 sphoBrium); sometimes showing contours both outside and inside 

 {Amceha protens). In the former case the inner zone adjacent to 

 the membrane shows a decreasing density inwards, until the linin 

 merges insensibh' into the intranuclear reticulum. In free-nuclei 



Fig. 29. — Vahlkampfia Umax; chromatin forming the nuclear membrane and 

 giving rise to chromidia. (After Calkins.) 



formation, antecedent to gamete formation described above, the 

 nuclear membranes 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 chromo- 

 meres indicates its presence (Fig. 29). 



One peculiarity of the nuclear membranes of Protozoa which dis- 

 tinguishes them from nuclear membranes of tissue miclei, 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, flaring 

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

 branes etc., during division, see p. 114.) 



(d) Plastin,— Plastin, perhaps not dift'erent from linin, has been 

 definitely identified only in a few cases of Protozoa, and much 

 remains to be done before an accurate account of its functions in the 

 nucleus can be written. Probal)ly a deri\'ative of chromatin as 

 early suggested by van Beneden, it is an important substance in 



