GROWTH AND DIVISION 87 



Division, but without separation of the products, can even 

 occur though both nucleus and primordium are excised before 

 there is any visible beginnings of the fission line (Fig. 19c). 



Total removal of the macronucleus as early as stage 4 does not 

 always preclude division even though complete development of 

 the primordium cannot occur without nuclear support. However 

 necessary for the original stimulus to divide, the macronucleus, as 

 Yagiu (1951) also found in Condylostoma, does not seem to be the 

 immediate trigger for the actual process of fission. (Micronuclei 

 are of course not concerned, because Schwartz (1935) showed that 

 growth and division occur in stentors from which these nuclei 

 have been removed.) At stage 5, when the division line is presum- 

 ably being determined, the cell can be cut through with a glass 

 needle following exactly the path which the furrow will take, or in 

 stage 6, when the furrow is visible, it can be slashed through with 

 the needle around its entire course, and still division is completed. 

 Together, these operations indicate that the division furrow is not 

 a structural elaboration ; for if it were, the nucleus would presum- 

 ably be indispensable for the synthesis of new parts, and because 

 any specialized ** organelle of division " would be destroyed by the 



A. a : Left half of stage-6 divider rotated in polarity reverse 

 of that of right half, with no fission line yet begun, b, h' : 

 Furrows develop along predetermined course in both halves, as 

 seen in ventral and dorsal views, but not joining, c: Hence 

 daughter cells held together by cytoplasmic connections, only 



later pulling apart. 



B. S. coeruleus continues division and anlage develops com- 

 pletely though stage-3 primordium shifted to the posterior end. 

 a: The operation; b: resulting arrangement; c: fission 



consummated. 



C. Continued fission of stage-6 divider after both nucleus 

 and primordium removed before any sign of cutting of stripes 

 to form the fission line, a: The operation, excision of the 

 anlage patch and clumped macronucleus. b: Division nearly 

 completed but products held togethe-r by a cytoplasmic bridge 

 probably due to cortical pattern disturbances from cutting. 

 c: Substance of opisthe now largely absorbed into the proter. 



D. Tail-to-tail heteropolar grafts easily pull apart; but even 

 head-to-head pairs like this, in which the heteropolar striping 

 does not join, also can separate or "divide" neatly by a course 



which was not observed. 



