MORPHOGENESIS 809 



case comes from another phase of development in Zoothammum. In the 

 sexual process, the apical cell becomes the sessile macrogamont. The 

 fusion of a free-swimming microgamont with the sexually differentiated 

 apical cell arrests axial development for several days, pending the origin 

 of a new apical cell from one of the ex-con jugants. In the meantime, 

 all of the cells on three or four of the youngest (uppermost) branches 

 begin to divide precociously. The terminal branch cells are aroused 

 to unusual mitotic activity, producing twice as many generations as when 

 they comprise a part of the vegetative colony. The common branch cells 

 are likewise activated to produce secondary and even tertiary branches. 

 This precocious development never occurs when the apical cell presides 

 over a vegetative colony. Neither does it occur as a result of decapita- 

 tion — when the apical cell is destroyed. The phenomenon appears to be 

 initiated by qualitative changes in the coordinating mechanism, which 

 arise in consequence of reorganizational activities in the single apical 

 cell. 



The growth relations are likewise altered by conjugation in Z. arbus- 

 cula (Furssenko, 1929). Each of the several primary axes in this species 

 bears an apical cell which becomes the macrogamont during the sexual 

 period. Conjugation on one axis stops further apical extension of that 

 branch until two new vegetative axes spring from the two "stem cells" 

 of the fourth ex-conjugant generation. One daughter cell from each of 

 the first four generations differentiates into a very large macrozodid 

 (immature ciliospore) . A single conjugant therefore produces two grow- 

 ing points and a cluster of from four to six bulbous macrozooids. As 

 is the case in Z. alternans, several of the small secondary branches below 

 the conjugant-bearing node develop to the proportions of primary axes. 

 Under the influence of a vegetative apical cell, these branches do not 

 hypertrophy. 



Furssenko accounts for the changed relations between apical and 

 subordinate regions in terms of local variations in the food-energy re- 

 quirements. In the light of the above observations, he supposed that 

 the cluster of huge non-feeding macrozooids at the tip of the stalk, to- 

 gether with the two developing apical cells, have energy needs in excess 

 of the apical requirements in non-conjugating colonies. Multiplication 

 of the actively feeding cells on neighboring branches presumably occurs, 

 in order to compensate for the unusual metabolic needs at the apex. 



