228 The Sarcodina 



living Mycetozoida in which the mature stage is a migiatory plasmodium; 

 more or less complex sporangia are produced in many genera. 



Suborder 1. Acrasina. In this group, a small uninucleate "myxamoeba" 

 is released from the cyst ("spore"). Sexual phenomena have not been 

 demonstrated. These myxamoebae (Fig. 5. 20, A) lead an active life, 

 feeding typically on bacteria and undergoing fission. Under certain con- 

 ditions, which include a favorable humidity (Hi/) and perhaps partial 

 exhaustion of food (117), a pseudoplasmodium is developed by the ad- 

 hesion of myxamoebae to one another (Fig. 5. 20, B). In spite of its 

 organization, the pseudoplasmodium of Dictyostelium discoideiim moves 

 as a polarized vmit (116) and may grow by fission of the component 

 myxamoebae. The myxamoebae are said to cease feeding after formation 

 of the pseudoplasmodium in Dictyostelium (137) and the aggregate ap- 

 parently is a preliminary step toward sporulation. 



Sporulation in some of the simpler Acrasina, such as Guttulina, in- 

 volves merely a heaping up of the m)T{amoebae into a compact mass and 

 then secretion of a cyst membrane (117). In such specialized types as 

 Dictyostelium discoideum (11), a pseudoplasmodium, under favorable 

 conditions, may first vmdergo a certain amount of migration. At sporula- 

 tion, the pseudoplasmodium gradually assumes an upright position and 

 becomes reorganized into a pseudosporangium (Fig. 5. 20, C-I). During 

 the late migratory phase, the posterior components of the pseudoplas- 

 modium are differentiated into intensely staining pre-spore cells; the 

 anterior units become stalk-cells; those at the base of the pseudoplasmo- 

 dium, basal-disc cells. Later on, the pre-spore cells are transformed into 

 spores. Morphogenesis also involves changes in position of the units. The 

 stalk-cells most anterior in the migratory stage are pushed up to and over 

 the top of the stalk-sheath and do^vn toward the basal disc during de- 

 velopment of the pseudosporangium. As a result, the relative positions 

 of various groups of cells are reversed (Fig. 5. 20, K-M). The pseudo- 

 sporangia are quite specialized also in Polysphojidylium. One interesting 

 feature of this commimal process is that sporulation follows a specific 

 pattern. Even after being crushed and mixed together, pseudoplasmodia 

 of two different species may reorganize and then produce their typical 

 spore-bearing structures (118). 



The best known genus is Dictyostelium Brefeld, species of which have been investi- 

 gated in detail by several workers (II, 114, 116, 137). Certain species of Dictyostelium 

 have been maintained in cultures (12, 25, 115, 116, 137). Other genera (102) include 

 Acrasis Van Tieghem, Coenonia Van Tieghem, Guttulina Cienkowski, GuttuUnopsis 

 Olive, and Polyspliondylium Brefeld. The status of Sappi7jia Dangeard, sometimes 

 included in this group, is uncertain (117). 



The Acrasina are free-living forms found commonly in soil and on decaying wood, 

 leaves, and straw, and all of them apparently feed on bacteria. 



Suborder 2. Plasmodiophorina. These organisms invade cells in the 

 roots and underground stems of higher plants. Infections are often ac- 



