RHIZOPODS, ACTINOPODS, SLIME MOLDS, SPOROZOA 79 



Palade particles. The latter are sparsely distributed also in the 

 cytoplasmic matrix — an observation that may be correlated with 

 the generally diffuse basophilia of the large amebae. The smaller 

 amebae tend to have more abundant particle-studded membranes 

 and more basophilic cytoplasm. 



Cytoplasmic inclusions generally recognized by light micro- 

 scopists in the giant amebae and characterized by their stratification 

 in centrifuged specimens as well as by their staining reactions 

 include the following: 



(1) Tiny, non-staining alpha granules, not mentioned by 

 electron microscopists except Cohen (1957), who believed that 

 they were represented by small, dense particles enclosed in 

 membranes; their nature and significance remain unknown. 

 Mercer (1959) saw similar small granules but did not attempt to 

 identify them. 



(2) Beta particles, larger than the alpha particles, now well 

 identified as mitochondria. These are of the conventional micro- 

 tubular type and show no special properties except for the pecu- 

 liarly patterned mitochondria found in Pelomyxa during mitosis 

 by Pappas and Brandt (1959) and described in the preceding 

 chapter. The halo of mitochondria about the contractile vacuole 

 has also been noted. 



(3) Refractile bodies, unidentified in electron micrographs. 



(4) Contractile vacuoles, discussed in Chapter 2. 



(5) Plate-like or bipyramidal crystals in vacuoles. These are 

 clearly recognizable in electron micrographs as rather large 

 vacuoles of irregular shape within which a polygonal empty space 

 appears. The crystals apparently survive fixation and embedding 

 but either are torn out of the embedding plastic during sectioning 

 or are quickly evaporated under the electron beam. Analysis of 

 isolated crystals from both Amoeba and Pelomyxa has recently 

 been reported by Grunbaum, Moller, and Thomas (1959) and by 

 Griffin (1960), using chemical, physical, and optical methods. The 

 crystals are identified as carbonyl diurea or a closely related 

 compound; slight differences in composition or the presence of 

 additional substances in small amounts may account for the 

 different characteristic crystal shapes. The crystalline substance is 

 believed to be a nitrogenous excretion. 



Membrane-enclosed fat droplets are often seen in the cytoplasm. 



