MORPHOGENESIS 807 



former remaining in the terminal position on the laterally growing 

 branch axis. Thus the primary developmental functions of a growing 

 colony are almost exclusively limited to the terminal cells of the primary 

 and branch axes (Fig. 185). 



A painstaking cytoiogical analysis of normal development in this 

 species led Faure-Fremiet (1930) to postulate that the two daughters 

 resulting from the division of one initial cell are never equivalent as 

 to their developmental potentialities. He assumed that quantitatively 

 differential divisions of the apical cell series restrict the subsequent power 

 of division in the branch strains; and, similarly, the qualitatively differ- 

 ential nature of the first division of the initial branch cells effects a 

 segregation of potencies for ciliospore formation. Such a hypothesis of 

 embryonic segregation by division apparently covers the facts of normal 

 development. A terminal brancli cell, for example, produces fewer gen- 

 erations than the apical cell (see Fig. 185). And, for the most part, 

 a ciliospore differentiates only from the lateral cell of the first branch 

 generation or from its two immediate descendants on certain of the 

 branches. 



The facts derived from regeneration studies are not in accord with 

 Faure-Fremiet's assumptions. Common branch cells above or lateral 

 to the supposedly differential divisions retain, for a time at least, po- 

 tentialities for regenerating large portions of the colony. An apical cell, 

 terminal branch cells, common nutritive cells, ciliospores, and sometimes 

 gamonts differentiate at appropriate positions on the regenerate. Further- 

 more, the ciliospore-forming cells can be induced to differentiate as new 

 apical cells, which continue axial development according to the normal 

 pattern (Fig. 186). 



If the well-defined apical cell is removed, the terminal cell of the 

 topmost branch usually differentiates as a new apical cell, the subse- 

 quent development of which is identical with that of the original. 

 But if the apical cell and the first terminal branch cell are destroyed, 

 the functions of the former are assumed by either the subterminal cell 

 of the topmost branch or the terminal cell of the penultimate branch, 

 more frequently the latter. A variety of operations performed by the 

 writer (1938b) upon large and small colonies show that subordinate 

 cells — terminal branch cells or merely the common nutritive cells, the 

 complete developmental potentialities of which are never otherwise 



