PROTOZOA AS CELLS 57 



and according to the same fundamental rules as do other eucells. 

 This is hardly surprising; ancestral protists presumably made 

 the rules that metazoa and metaphytes have been following ever 

 since, and exploratory variants of these rules have persisted and 

 been embellished during the long evolutionary histories of 

 divergent protistan groups. As many inspired light microscopists 

 have realized, study of protozoan karyological pecularities is not 

 merely esoteric. 



Most electron-microscope studies of protozoa have included at 

 least incidental observations on nuclear structure. We can in a 

 general way list some properties that appear to be common to 

 most of them, and then discuss the more spectacular exceptions. 

 Our most consistent information concerns the organization of 

 the nuclear envelope. 



Surrounding the nucleus of all eukaryotic cells is an envelope 

 consisting, in every case rigorously examined, of two distinct 

 membranes separated by a low-density zone. The overall thickness 

 of the envelope generally is in excess of 20 m^ and may be twice 

 that or more, since the light layer or space varies considerably in 

 width (Fig. 16, PI. IV). A particularly elegant example of nuclear 

 membrane structure is that revealed in Grimstone's (1959c) study 

 of Trichonympha. The nuclear envelope here is perhaps thicker 

 than in many other cells but its structure is probably typical. The 

 two membranes of the envelope are each about 7 m/x in thickness 

 and the intervening space averages 23 m^. Distributed fairly 

 regularly over the envelope are pore-like structures, or annuli, 

 whose rims are formed by a fusion of the outer and inner envelope 

 layers. Frequently, a layer of moderate density occupies the 

 opening of the pore and may constitute a septum or plug across it. 

 In surface view, the rims of the pores appear quite dense, partly 

 owing to the presence of ten to 12 small dark granules regularly 

 disposed around them; these do not appear to represent fibrils 

 or cylinders, but rather distinct particles lying on the outer 

 surface of the membrane. About 80 pores per /x 2 occur in 

 Trichonympha; their inner diameter is approximately 45 m/x. 



In many instances among multicellular organisms, and in some 

 protozoa (Sager and Palade, 1957) continuities may be seen 

 between the outer nuclear membrane and cytoplasmic membranes 



