58 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



(Fig. 16, PL IV), the peri-nuclear space thus being in communica- 

 tion with cavities of the endoplasmic reticulum. In some species 

 of the flagellate Chrysochromnlina, the outer nuclear membrane is 

 continuous with the outer chloroplast membrane where the 

 pyrenoid abuts against the nucleus (Manton and Leedale, 1961). 

 Whether these continuities reflect a persistent functional relation- 

 ship or not we cannot say. It is clear that in many metazoan cells 

 the nuclear envelope forms during late mitosis from a halo of 

 membranous reticulum (e.g., Barer, Joseph and Meek, 1959; 

 Harris, 1961). The conclusion that cytoplasmic membranes, 

 particularly the RNP-rich granular membranes, may originate at 

 the nuclear surface has been tempting; however, to date no 

 evidence of a clear gradient in concentration of Palade particles 

 or membranes from the nucleus toward the cell periphery has 

 been presented, even in cells in which active regeneration of 

 ergastoplasm is known to be occurring (Bernhard, 1959). 



Grimstone (1959c) has considered his micrographs of Tri- 

 chonympha in the light of these data. He finds in the cytoplasm 

 many elongate, flat membranous sacs bearing granules on their 

 outer surfaces; no reticular interconnections appear. These are 

 particularly abundant around the nucleus and may form a dis- 

 continuous shell over its surface. However, no direct continuities 

 between nuclear and cytoplasmic membranes are seen, and the 

 latter are considerably thinner than the former and also lack their 

 annular structure. Grimstone illustrates clusters of unattached 

 granules very close to the nuclear membrane, and he considers 

 it likely that the membranes may be formed in the near vicinity 

 of, but outside, the nucleus, and that the granules then are 

 acquired by migration from the nuclear surface. 



The majority of protozoan nuclei studied probably have 

 envelopes similar to that of Trichonympha, although perhaps the 

 annuli are less abundant (Fig. 17, PL V). Opinion differs as to 

 whether the pores are patent. A striking elaboration of the nuclear 

 envelope was found in Amoeba proteus first by Harris and James 

 (1952) and Bairati and Lehmann (1952), and subsequently has 

 been studied by other workers (see citations on amebae in 

 Chapter 3). A porous double membrane exists as in other nuclei; 

 immediately beneath this is a layer, up to 300 mju deep, consisting 

 of an orderly honeycomb arrangement of packed tubes (Figs. 18 



