PROTOZOA AS CELLS 61 



double nuclear envelope. The similarity of structure is unques- 

 tionable ; the tubes in Endamoeba are somewhat more slender and 

 perhaps not so regularly packed as those of Amoeba, but like the 

 latter their walls appear more membranous than fibrous, and the 

 layer is about 450 m/x deep. The same concentric arrangement of 

 tubes about the pores of the nuclear envelope obtains mEndamoeba, 

 but the tubes lead outward and open into the cytoplasmic matrix. 



No figures are given for the dimensions of the gregarine nuclei, 

 but it may be presumed that they are fairly large. Nuclei in the 

 feeding gregarines are known to undergo considerable volume 

 increase as the cell grows; measurements from a phase-contrast 

 micrograph in a later paper of the Beams group (1959a) indicate a 

 nuclear diameter of at least 20 fi. In Endamoeba blattae, the nucleus 

 is up to 20 /x in diameter. Other than their size, which certainly is 

 not unique, it is difficult to imagine what peculiar property in 

 these organisms could be associated with the presence of this 

 ordered structure. Electron micrographs of other gregarines do 

 not show details of nuclear membrane structure, but certainly in 

 Pe/omjxa and several small lobose amebae, no honeycomb layer 

 is present. The most striking fact emerging from these studies is 

 this : whatever the function of the honeycomb layer, and whether 

 or not it is homologous in the gregarines and the amebae, its 

 perforations coincide with annuli in the double nuclear envelope. 

 This indicates that the annuli have to be accessible to both 

 nucleoplasm and cytoplasm and suggests that they have particular 

 importance in some aspect of nucleo-cytoplasmic communication. 



Since protozoan nuclear contents vary widely in their appearance 

 in life or when stained for light microscopy, it is not surprising 

 that generalizations on electron microscope structure must be so 

 broad as to be practically meaningless. An "average" interphase 

 nucleus contains a matrix of finely granular or filamentous material 

 irregularly distributed throughout, with amorphous condensa- 

 tions often appearing peripherally or internally. Some of these 

 condensations represent true chromatin bodies while others may 

 be coagulation artifacts. Bodies that usually are larger, more 

 granular, and more compact often are identifiable as nucleoli or 

 as endosomes (Feulgen-negative bodies that probably are 

 analogous to nucleoli but that typically maintain their identity 

 through mitosis). 



