RHIZOPODS, ACTINOPODS, SLIME MOLDS, SPOROZOA 81 



clusters of dense, 20-m/x particles, often near food vacuoles. These 

 aggregate in large, rod-shaped masses showing a regular crystal- 

 line order; the rods may reach 10 ^ in length in the cyst. Cyto- 

 chemical studies by these authors revealed a high RNA content, 

 with some protein. Electron micrographs of RNA'ase-treated 

 cells showed the particles much flattened and linked together in 

 rows. Their significance remains enigmatic. 



Zaman (1961) reported that the characteristic uroid, or irregular 

 tail region, of Entamoeba invadens consisted of a convoluted cyto- 

 plasmic extension surrounded by dense vesicular material assumed 

 to be a mucoid secretion. Ejection of digestive residues and 

 excretion products into this mass was suggested. 



The remarkable nuclear structure of Amoeba proteus was dis- 

 cussed in Chapter 2. Pelomjxa nuclei lack the cortical honeycomb 

 layer but have, just below the nuclear envelope, a zone of varying 

 thickness containing a loose meshwork of fibrils 6 to 7 m/x in 

 diameter (Pappas, 1959). Both genera have intriguing helical 

 fibrils within the nucleus. The smaller amebae generally have 

 conventional-looking nuclei. The sparse fibrogranular nucleo- 

 plasm encloses clumps of chromatin and dense, usually peripheral, 

 nucleoli. Only in Endamoeba blattae has the honeycomb layer of 

 the Amoeba nucleus been found, but here on the external surface 

 of the envelope. Since the genus Endamoeba is noted for its thick 

 nuclear membrane in light-microscope preparations, other species 

 may prove to have the honeycomb surface also. 



In summary, it is evident that the free-living ameba, large or 

 small, has a full complement of the protoplasmic organelles that 

 are recognized as standard equipment for cells even of the highest 

 organisms. The absence of restrictive morphological differentia- 

 tions in the ameba has long endeared it to the experimentalist ; the 

 conventional nature of its cytoplasmic furniture may now reassure 

 him that his pet is not unreasonably. exotic. Such peculiarities as 

 various species possess represent variations of these conventional 

 structures that are more accessible to study than is the case with 

 many other cells because of the ameba's extraordinary docility 

 under surgery and other violent manipulations. 



The remainder of the great array of rhizopods remains almost 

 completely unexplored by electron microscopists. Most of them 

 construct tests or shells around their bodies, a habit that is carried 



