224 The Phenomena of Morphogenesis 



lishing a center. Other fusions occur later and are followed by meiosis. 

 The zygotes can be distinguished by their greater size. 



In the pseudoplasmodium the myxamoebae do not fuse but each cir- 

 culates freely among its neighbors, and the whole mass moves over the 

 surface of the substratum by means that are not yet clearly understood. 

 This body of separate cells, however, is not without some degree of differ- 

 entiation. It is elongated in the direction of its movement, which is toward 

 the light. The apical end is slightly pointed and lifted above the rest and 

 is richer in acrasin than the other regions. It is the part of the mass that is 



Q> 



^0 & 



X 0' f 



°\o hy»* 



°"(pfc 



on " ^ ° 



^0 e- <g q POQtflQ 



<fe>„ 



-3 « (7 fc *> 



9 

 9 e 





o° 8 ° !) 



V 





'* 



Aa ^ 





M 





(7 



n 



LJ>o<»*~» «% 



li 



**. 



'i 



100// 



Fig. 8-23. Dictyostelium. Stages in aggregation of myxamoebae into a pseudoplas- 

 modium. ( From J. T. Bonner. ) 



sensitive to the stimulation of light and seems to serve as a directive 

 center for the whole. A pseudoplasmodium from which the apex has been 

 removed will stop its motion and settle down at once to form a fruiting 

 body. 



Two groups of cells may be distinguished in the pseudoplasmodium. 

 Those near the apex and destined to form the stalk of the sorocarp are 

 somewhat larger than the ones in the posterior region, which will later 

 form spores. The proportion between these two types is maintained by 

 a regulatory process irrespective of the size of the whole mass. Some cell 

 division continues in the pseudoplasmodium but the rate is different in its 



