ORIGINS OF AGAMIC PATTERNS 637 



and Dictyostelium, have received considerable attention /^ The following 

 account is based on the recent papers of Harper and Arndt. Cultured in 

 dung decoctions with agar or agar-bouillon and Bacillus coli, the spores 

 give rise to small amoeboid cells (myxamoebae) ; these move about in- 

 definitely, feed and divide for a time, but sooner or later begin to move 

 in more or less definite streams, at first in all possible directions, constitut- 

 ing a network of moving amoebae. According to Arndt, appearance of 

 directed streaming of the amoebae is associated with lack of nutrition in 

 the culture. As the streaming continues, centers of aggregation appear, 

 apparently entirely by chance, except that they are more likely to form 

 in dryer parts of the culture — for example, at the margins (Harper). 

 Harper and Arndt agree, however, that the amoebae react primarily to 

 a condition produced by other amoebae rather than to local differences 

 in substrate otherwise produced. Streaming of amoebae in the vicinity 

 of an aggregation becomes directed toward it; and as the aggregation in- 

 creases in size, the streaming becomes increasingly definite and the area 

 of centered streaming greater. Evidently the differential to which the 

 amoebae react represents a radial gradient system of variable scale, pre- 

 sumably due to diffusion of a substance produced by the amoebae. 

 Early-stage aggregations may be only temporary and may disappear com- 

 pletely by alteration in direction of reaction of the amoebae, or an ag- 

 gregation may change its position (Arndt). But once an aggregation is 

 well established, the amoebae continue to stream toward it and build it 

 up above the surface of the nutritive substrate, forming the beginning of 

 an axis. The further development of Polys pondylium, as described by 

 Harper, is of great interest. Figure 204, ^, is an approximate representa- 

 tion of the chief paths of streaming of amoebae toward a center, as shown 

 in a photomicrograph by Harper (1929). The developing axis, rising above 

 the substrate, is indicated at a in the figure. The amoebae continue to 

 arrive at the center of aggregation, move up the axis in or on the slime 

 secreted by those already there, and come to rest at the top, forming 

 an elongated cylindrical structure, the sorogen. From the basal part of 

 the aggregation a slender stipe forms by transformation (differentiation?) 

 of amoebae into cells resembling plant parenchyma with complete cessa- 

 tion of amoeboid activity. The stipe elongates by continued transforma- 

 tion of amoebae into stipe cells at its distal end. The resulting form is an 



"Brefeld, 1869, 1884; van Tieghem, 1880; Olive, 1902; Potts, 1902; von Schuckmann, 

 1925; R. A. Harper, 1926, 1929, 1932; Arndt, 1937. 



