GROWTH AND DIVISION 



8l 



Fig. 1 8. Experimental conditions affecting location of the 

 division furrow. 



A. Head only of stage-4 divider excised; furrow is shifted 

 forward with result that proter is only half the volume of the 

 opisthe. Proter begins regeneration only after fission is 



completed. 



B. Oral side of a coeruleus grafted transversely to animal from 

 which anterior end was excised. Unusual subsequent dividing 

 off of an anucleate product without an oral primordium {x) 

 indicates how extensively process of division may be upset by 



m i sarrangements . 



By centrifugation, Popoff (1909) was able to produce unequal 

 fissions in coeruleus. The macronucleus was also unequally divided, 

 yielding, for example, a small cell with 3 nodes and a larger one 

 with 1 6. Presumably the level of the fission line was located other 

 than normally, though no details w^ere given. Prowazek (1904) 

 likewise reported unusual cases of shift in the fission level leading 

 to unequal daughter cells, as did Packard (1937), without being 

 clearly aware of what he was observing. Altogether, these studies 

 show that the fission line becomes fixed beyond altering only late 

 in division, and can be shifted in its location by earlier influences. 



4. Incitement to division 



No one yet knows what causes a cell to divide and Stentor is no 

 exception. In all the experiments on stentors by myself and others 

 no operation has been established as promptly and invariably 

 leading to cell division. Yet the search for the inciting cause is so 

 important that it is appropriate to discuss the few eflForts that have 

 been made in this direction with stentors. Generally, stentors 

 attain a certain maximum size before dividing, i.e., dividers are 



