6o2 H. S. Jenjungs 



left behind in the growth of the tip of the bodv, so that in the adult 

 infusorian it lies halfway back to the middle of the animal (Fig. 3, ^). 

 At the next fission it of course goes to the anterior product, lying 

 at or behind its middle. By the greater grow^th of the anterior 

 end it is further displaced backward, so as to lie clearly behind the 

 middle. At the next fission it must then go to the posterior prod- 

 uct, and be near its anterior end. Now it is again displaced 

 slowlv backward, the same processes being repeated. Thus the 

 process is normally one of steady movement backward, interrupted 

 by fissions which at intervals leave the spine near the anterior 

 end of the posterior individual. A diagram showing this normal 

 course of events is given in Fig. 7. 



Sometimes through irregularities in growth, or other cause, the 

 structure comes to be situated very near to or at one end (as in 

 Fig. 4, *-i--^^"--). Then the course of events becomes slightly 

 different. If the structure is near the posterior end (Fig. 4, *) the 

 posterior tip grows back from it only a little, so that it still remains 

 behind the middle of the body. At the next fission it therefore goes 

 to the posterior individual (as it would in the "normal" course). 

 Xow the posterior end again grows back but a little, while the 

 anterior tip grows much, so that the spine is still behind the middle. 

 It therefore goes again to the posterior individual. It may thus re- 

 quire as many as three fissions to bring the structure to the middle, 

 so that it passes again to the anterior individual, reestablishing 

 the alternations (Fig. 4, ^^°^-), 



Is situated at or verv near the anterior tip, the structure is car- 

 ried for%vard in the growth processes; it may therefore remain for 

 several generations in this region (Fig. 4,^^*°^^), before it is dis- 

 placed backward sufficiently to lie behind the middle. Possibly 

 a structure might in the course of time attain a permanent posi- 

 tion at the anterior tip. This seems indicated by the last three 

 generations of a. 



Thus on the whole the general tendency of the growth processes 

 is to shift any surface structure from the ends toward the middle 

 of the body, while the fissions again transfer it toward one end; 

 with the further result of an alternation of position from the 

 anterior to the posterior product of fission and back again . 



