ORIGINS OF AGAMIC PATTERNS 639 



elongated cylindrical mass of amoebae borne on a slender stipe. This dif- 

 ferentiation into stipe and sorogen perhaps results from arrival of amoebae 

 at the summit of the aggregate more rapidly than they are transformed 

 into stipe cells, so that conditions in the mass at the summit become dif- 

 ferent from those in the developing stipe and perhaps retard the transfor- 

 mation. 



Branching in Polys pondylium occurs as follows: At a certain stage a 

 constriction appears near the base of the cylindrical sorogen, and a mass 

 of amoebae is "left behind" on the stipe as it elongates further. In Fig- 

 ure 204, 5, three such masses have been isolated, and a fourth is constrict- 

 ing off at the base of the sorogen. Each of these masses on the stipe gives 

 rise to branches, usually a whorl. The branches develop in much the same 

 way as the main axis but at an angle to it; that is, the amoebae now 

 react plagiotropically. In Figure 204, C, D, and E show fully developed 

 individuals — unbranched (C), with two whorls (D), and with seven 

 whorls (£). Development of Didyostelium is, in general, similar except 

 that it usually does not branch. In these forms cell division is apparently 

 limited to the earlier stage of feeding and undirected movement of sepa- 

 rate amoeboid cells and has not been observed during morphogenesis. 



At no stage in this development do the amoeboid cells form a Plas- 

 modium, as in myxomycetes. They stream and aggregate as separate 

 cells, and even the cylindrical sorogen consists of cells capable of resuming 

 amoeboid movement as separate cells, if again brought into contact with 

 the culture substrate, and of giving rise to a new sorogen. 



As regards interpretation of the origin of this axiate pattern, Harper 

 holds that there is no reason "for assuming any superchemical or physical 

 stimuli or the action of organizing or regulating principles associated 

 with the multicellular organismal condition resulting from the aggregation 

 of the myxamoebae. The behavior of the pseudoplasmodium as a whole 

 is the sum of the behaviors and reactions of the individual myxamoebae" 

 (Harper, 1929, p. 237). Arndt does not go beyond the assertion that the 

 culture of myxamoebae is a harmonious-equipotential system. 



While no data are at hand concerning the presence or absence of a 

 gradient in the developing axis, there is every reason to believe that a 

 gradient is present. Amoebae are being continuously added at the free 

 end; and when they arrive there, they are obviously in a different condi- 

 tion from earlier arrivals which have lost motility and have come to re- 

 semble ordinary plant cells. The stipe develops progressively as the amoe- 



