-78 THE BIOLOGY OF STENTOR 



permits normal fission as the division line crosses the suture, if 

 other ectoplasm from the same dividing animal is shifted to fill 

 the gap the furrow then does not cross over. Evidently, as Weisz 

 remarked, the path of the fission line is strongly and uniquely 

 determined and cannot be initiated by local point-to-point processes 

 alone. 



Fixity of the presumptive furrow is shown by the observation, 

 first reported by Johnson, that if the anterior or posterior end of a 

 divider is cut off just before the furrow is to appear, unequal 

 daughters are formed because the fission line appears in its normal 

 place. If at the same stage a cell is prematurely divided by cutting, 

 fission still occurs along the predetermined oblique path half of 

 which lies in each fragment with the result that small blebs are 

 separated (Fig. i6c). An artificial cutting of the body stripes is thus 

 not used as a substitute for the normal fission Une. Also, if pre- 

 furrow dividers are cut through transversely, first on one side and 

 then on the other so that the two halves remain fused together, 

 division still occurs but it is oblique and not in the line of heal. 

 Still more convincing is the experiment in which these cut halves 

 are rotated 180° upon each other so that the body striping does not 

 match or heal together, as evidenced by obvious discontinuities in 

 the granular stripes; for even in this case relatively normal and 

 equal division occurs with both parts of the severed primordium 

 going to the opisthe and division was obviously not in the line of 

 created discontinuities (Fig. i6d). 



These cutting experiments not only attest the fixity of the fission 

 line, even before it is visible as such; they also strongly indicate 

 that division in stentor is not due to ingrowth of separating mem- 

 branes, surface tension changes, or other mechanisms which have 



B. Stage-5 divider grafted to a non-divider. Fission continues 

 on the divider side but furrow stops when it meets indifferent 

 striping. Daughter cells do not separate because held together 



by the partner of the graft. 



C. Stage-4 divider cut in two continues fission along the pre- 

 determined fission line resulting in separation of small blebs. 



D. Stage-4 divider with anterior rotated on posterior half, 

 separating the primordium into 2 sections. Both parts of the 

 anlage still go to the opisthe. Fission line does not follow the 



discontinuity of striping at the suture. 



