88 THE BIOLOGY OF STENTOR 



Operations just described. Instead, it appears again that furrow 

 formation involves only the severing of longitudinal structural 

 elements of the ectoplasm, v^hich was merely hastened or abetted 

 by the needle. Constriction is probably due to the action of contrac- 

 tile elements largely already formed. 



Cutting of stripes alone does not result in fission however. The 

 ectoplasm of non- dividing cells can be completely cut through 

 around the equator and division never follows, the cortical struc- 

 tures merely healing together, often without leaving any indication 

 of the operation. Conversely, in abnormal situations one some- 

 times finds stentors in which complete furrowing seems to have 

 occurred but division does not follow. These cases indicate the 

 importance of constriction in division, for it is almost certain that 

 had constriction occurred, this type of specimen would have been 

 divided in two. 



Division is not the only means by which two separate stentor 

 individualities can become separated. Fusion masses of two or 

 more stentors show a strong tendency for the components to pull 

 apart. This is especially the case in the heteropolar pairs, whether 

 joined by the heads (Fig. 19D) or the tails; for when stripes of 

 opposite polarity meet it is quite evident that they do not join and 

 at this locus of discontinuity a separation may occur. Weisz (1951a) 

 remarked that separations of tail-to-tail telobiotics are ''strikingly 

 reminiscent of vegetative division"; yet they are different in that 

 pulUng apart requires a long time for completion, as if sharply 

 localized constriction could not occur at all. 



By growing coeruleus in what were probably rather putrid 

 cultures of beef extract, Stolte (1922) produced animals with highly 

 vacuolated endoplasm which showed many anomalies of division. 

 Animals could divide into three parts instead of two, producing 

 posterior daughter cells without primordia or nuclei, or showed 

 very unequal divisions yielding abnormally small opisthes. A case 

 of partial vertical fission was even described which resulted in an 

 animal with two holdfasts. Since the conditions were obviously 

 abnormal, the interpretations offered seem dubious; yet these 

 observations suggest, as Weisz (1956) proposed, that the endo- 

 plasm is important in division, vacuolization greatly disturbing 

 whatever its function may be. 



Postponed fission, in which division is much delayed but eventu- 



