RELATION OF RHYTHMS AND ENDOMIXIS. 4! 



the intermediate points of the "rhythms" in this curve. The 

 same is true of other curves in which an alternate rise and fall 

 in the division rate over long periods is not evident. Take for 

 example the graph of line M in Fig. 12 (/. c., p. 458). Here there 

 is no relation whatever between absolute division rate and 

 endomixis, the process occurring four times during division 

 periods of 2 -}- per day and five times when the division rate was 

 2 per day, the only relation between endomixis and division 

 tates being evident in the minor fluctuationf or "rhythms." 



In view of these results, I cannot find any constant correlation 

 between endomixis and division rate in Paramecium. 



Why then is it that endomixis is usually, though not always, 

 accompanied by a temporary reduction in the division rate? I 

 believe the latter to be consequent to the former. During en- 

 domixis profound chemical and physical changes are taking 

 place in the cell, of whose nature we are at present ignorant, but 

 which must necessarily exercise a great influence on metabolism 

 and growth, and would naturally tend to temporarily diminish 

 the division rate. Those exceptional cases in which endomixis 

 occurs at a high, or rising point in the division rate may perhaps 

 be attributed to some extrinsic stimulus in the culture medium, 

 tending to reverse the usual sequence of events. 



In his various papers on the subject of "rhythms" in Protozoa, 

 Woodruff maintains a more or less regular periodicity in their 

 occurrence (25-30 days or 40-50 generations in Paramecium 

 aurelia and 50-60 days or 8o-ico generations in P. caudatum). 

 In his most recent paper ('i7&, p. 461) he states that "General 

 changes in the environment of the animals ... do not per- 

 manently modify the length of the rhythm or the time between 

 successive endomicitic periods which is chaiacteristic of the 

 species." But (p. 462) "it has been found that the generation 

 periodicity . . . may be modified to a considerable degree by the 

 culture conditions which lower the division rate. In other words, 

 the rhythm appears to be more susceptible of modification in 

 regard to generations than time. As has been previously noted, 

 this is a surprising result, since a profound reorganization process 

 such as endomixis must be closely related to the general metab- 

 olism of the cell and this is expressed to a large extent in growth 

 and reproduction." 



