140 SENESCENCE AND REJUVENESCENCE 



which at first extends almost longitudinally. After these changes 

 have occurred the elongated moniliform meganucleus, mg, under- 

 goes concentration to a spherical form, as in Fig. 45, and the new 

 peristomial band of cilia gradually assumes a curved outline. Then 

 a transverse constriction appears in the meganucleus, which defines 

 two approximately equal halves, and this is followed by elongation of 

 the meganucleus (mg, Fig. 46), but separation of the two halves does 

 not occur until later. As regards the micronuclei, of which there 

 are usually a large number in Stentor (Johnson, '93), it is not known 

 whether all or only a part of them divide in each fission. The new 

 peristomial band of cilia changes its position, becoming more nearly 

 transverse and semicircular in outline (Fig. 46), and a mouth 

 begins to develop at its posterior end. This change in shape is 

 accomplished by a lateral outgrowth on one side of the body near 

 the middle which represents the anterior end and the peristome of 

 the posterior daughter individual. Just anterior to this develop- 

 ing peristome the level at which separation will occur is now indi- 

 cated by a constriction, as in Fig. 46. Other changes, indicated in 

 Figs. 46 and 47, consist in the further development of the new per- 

 istome and its continued approach to the transverse position, the 

 deepening of the constriction between the two individuals, and the 

 breaking up of the meganucleus into the characteristic segments, 

 beginning at the two ends. Still later the meganucleus separates at 

 the level of the cytoplasmic constriction, which continues to become 

 deeper, until the anterior member of the pair is attached to the 

 peristome of the posterior member only by a slender peduncle. 

 This finally separates and the process of fission is completed. As 

 regards the essential features of the process of fission, other species 

 of ciliates resemble Paramecium and Stentor, but the details of recon- 

 stitution differ for each species. 



The process of fission in these forms has been described at some 

 length because it is evident that it is a much more complex process 

 than ordinary cell division in the metazoa. So far as the cyto- 

 plasmic structures are concerned it is manifestly a process of recon- 

 stitution resembling that which occurs in agamic reproduction in 

 nature and in isolated pieces in the flatworms and many other 

 metazoa. Moreover, the process differs in the two forms. In 



