No. 3] MORPHOLOGY OF THE STENTORS. 503 



tion at the anterior end of the posterior zo5id, and almost 

 encircled by the adoral zone. The peristome, if it persist at 

 all, must remain adjacent to the adoral zone on its inner side 

 (right side during the earlier stages of fission, in which it 

 occupies exactly the same place as in Spirostomum), and, 

 therefore, in the precise position of the light band. 



The area of body-surface partially enclosed by the adoral 

 zone and destined to become the frontal field of the posterior 

 individual, undergoes little change up to the time of the forma- 

 tion of the pharynx (Fig. 27, /i). At this stage its stripes, 

 already narrower than those outside the ramifying zone, be- 

 come very much reduced in width, presumably by intercalation 

 of new "clear stripes" (Fig. 28, /J); but this I have not been 

 able to prove by direct observation. As fission advances, the 

 stripes of the new frontal field become narrower and more 

 numerous, thus approaching the condition of the old frontal 

 field (Figs. 33, 35, 36, /I). The relative width of the frontal 

 stripes to those at same plane is clearly shown in a transverse 

 section (Fig. 8,/i). 



The phases thus far described, although preliminary to the 

 actual fission, are extremely important, including as they do 

 the principal _/<?rwrt/zW act of the process — the development 

 of a new adoral zone. The gradual evolution of structures so 

 complicated as membranellas from a mass of indifferent proto- 

 plasm, is very striking. The fact that the membranellae are 

 derived from the outer layer of the endoplasm is not without 

 interest, showing as it does an ontogenetic development 

 different from the phylogenetic. For there is every reason to 

 believe that the membranellae were originally formed from 

 cilia or ciliary structures belonging to the ectoplasm. But the 

 ectoplasm is itself a modified layer, and hence not so well 

 fitted to give rise to a new and complex structure as the 

 undifferentiated endoplasm. It may further be noted that its 

 thickness is hardly sufficient to furnish material enough to 

 build up the membranellag. 



Another organ that is formed de novo very early in fission is 

 the contractile vacuole (Fig. 26, c.v.'^). At first it is always 

 smaller than the old contractile vesicle. In S. cceruleiis it 



