RHIZOPODS, ACTINOPODS, SLIME MOLDS, SPOROZOA 87 



suggesting pinocytic activity, and scattered empty vacuoles of 

 moderate size. 



Slime Molds 



Most textbooks of protozoology state that the slime molds 

 probably are not protozoa and then proceed to discuss them 

 anyway. Not to break with tradition, we shall do likewise. 

 Although in the formation of their multicellular fruiting bodies 

 they show marked affinities with the fungi, earlier stages in their 

 life cycle include flagellate or ameboid phases or both. In the 

 cellular slime molds such as Dictyostelium and Polysphondylium, 

 small amebae feed and divide repeatedly and eventually aggregate 

 to form a multicellular mass or pseudoplasmodium, which 

 typically assumes a sausage shape and subsequently migrates as a 

 whole, although individuals can change their position within the 

 mass. In the true slime molds or myxomycetes, such as Phjsarum 

 or Didjmium, flagellated stages may occur, followed by an ameboid 

 form that by fusion and by nuclear division and growth without 

 cytokinesis gives rise to a large, flat, spreading plasmodium in 

 which active protoplasmic streaming occurs through a reticulum 

 of veins or channels. A rhythmically alternating back-and-forth 

 flow in the channels is often seen; otherwise movement in the 

 solitary amebae and in the plasmodium is typically ameboid. 

 Pinocytosis has been observed in Phjsarum (Guttes and Guttes, 

 1960). The phenomenon of protoplasmic streaming in the 

 myxomycete plasmodium has attracted attention from students 

 of ameboid movement, and biochemical studies have led to the 

 extraction of a myosin-like protein, myxomyosin, by T'so, Bonner, 

 Eggman, and Vinograd (1956). 



Free vegetative amebae, aggregating stages, and pseudoplas- 

 modia of Dictyostelium discoideum are the subject of a study by 

 Gezelius (1961); the same species and Polysphondylium violaceum 

 have been studied by Mercer and Shaffer (1960). No differences in 

 cell structure are observed between solitary and aggregated cells. 

 The cytoplasm resembles that of the small rhizopod amebae, with 

 a rather dense fibrogranular matrix and vesicular elements of 

 varying shapes and sizes. Some flattened membranous sacs have 

 granular outer surfaces and in some individuals of Dictyostelium 

 these granular membranes assume the form of extensive lamellae 



