806 



MORPHOGENESIS 



transformed ciliospore produce, each time, an apical cell of the next 

 generation and a smaller lateral daughter, the initial cell of the presump- 

 tive branch at that node. The initial branch cells are strictly alternate 



Figure 185. A relatively 

 mature colony of Zootham- 

 nium alternans showing the 

 alternate arrangement of 

 branches and cells. The 

 apical cell of the nine- 

 teenth generation {TM. 19) 

 represents the growing 

 point of the primary axis. 

 The numerous common nu- 

 tritive cells of each branch 

 are division products of the 

 terminal cell of that branch. 

 On the seventh branch, 10^^ 

 designates the terminal 

 branch cell of the tenth 

 generation. The lateral cell 

 of the first branch genera- 

 tion (e.g., lA) or its 

 two immediate descendants 

 (e.g., IG^ and 1(7") repre- 

 sent the potential cilio- 

 spores; several stages in the 

 differentiation of these cells' 

 on several of the branches 

 are shown. (From Summers, 

 1938a.) 



in position along the primary axis, lying alternately on the right and 

 left sides of the axis at successive nodes. Cells of the branch strain de- 

 scend from the initial branch cell by a series of equal divisions. Here 

 again each division results in a median and a lateral individual, with the 



