GROWTH AND DIVISION 67 



then push up through the adjacent pigmented stripe somehow. 

 Apparently the new clear stripes with all their attendant complex 

 differentiations arise in situ, but of their origin we know nothing. 

 If we accept the genetic continuity of the kinetosomes and regard 

 them as fibrogenic granules which produce not only the cilia but 

 also the fibrils of the ribbon bundles in the clear stripes (see 

 Lwoff, 1950), the new kinetosomes will have to be traced to their 

 progenitors. 



Growth of other parts of the stentor cell present their own 

 special problems. The young daughter cell has a membranellar 

 band proportionate in length to its size. When full-grown the 

 length has increased and is still proportionate. Morgan (1901a) 

 therefore thought that this organelle grows in length and implied 

 that the number of membranelles increases. But if this were so, 

 Stentor would need two ways of producing membranelles : through 

 primordium formation and in situ. If the length of the membranellar 

 band is abbreviated by cutting, compensating growth does not 

 occur, contrary to a dubious observation of Stevens (1903). There- 

 fore it seems more likely that increase in length is accomplished 

 by the spreading apart of membranelles, already present, as 

 obviously occurs during the development of the oral primordium. 

 This point could be settled by counting the membranelles. Both 

 the total mass and the surface area of the macronucleus increase, 

 together or separately. Growth of the nucleus will be discussed 

 in a chapter devoted to that organelle. For the present, we may 

 merely remark that the trophic macronucleus, perhaps like the 

 giant salivary gland chromosomes of insects, represents a form of 

 nuclear material which adapts to the size of the cell and not vice 

 versa (cf. Goldschmidt, 1940). 



2. The course of normal division 



From regeneration studies we may say that a stentor could 

 multiply by simply cutting itself in two, the resulting daughters 

 then regenerating those structures which they lack. This would 

 be cell fission in the strict sense of the term and does occur under 

 unusual conditions of experiment. A stentor might even cut itself 

 into several fragments each of which would be viable, for neither 

 the whole nor the half represents a minimum unit of potential 

 organization. But in nature division is accomplished by trans- 



