Role of Preformed Structure in Cell Heredity 195 



meridians can produce progeny with only one. Their reproductive 

 autonomy is guaranteed only when the two oral meridians arc 90 or 

 more apart. That is why it has proved impossible to obtain and main- 

 tain multiplets of an order higher than quadruplets. It is also evidence 

 that important interactions and interferences occur when correspond- 

 ing replicate structures are unduly close together, suggesting — as we 

 shall later discuss more fully — that interacting gradients play an 

 important part in cortical morphogenesis and heredity. 



In addition to losses in the ways just described, new structures may 

 possibly arise de novo. For example, origin of a new oral apparatus 

 at a place where none preexisted possibly occurred as an exception, 

 although this is still clouded with uncertainties. It will be recalled 

 that we twice observed the origin of a whole new oral segment follow- 

 ing the "natural grafting"' of a piece of cortex from near the oral 

 region of a mate (page 181). Unfortunately, exactly what the grafted 

 piece contained remains unknown. Thus far the event has occurred 

 too rarely to be fully studied in its early stages. But it is clear that no 

 more than a short region of the oral segment, if any, could have been 

 grafted; and that from the graft developed an entire oral segment 

 with all five typical associated kinety fields, as well as vestibule, 

 mouth, and gullet. From the comparison of the behavior at fission 

 of complete and incomplete oral meridians given above (page 186), 

 we may be sure that, in order to do what it did, the graft must have 

 contained an essential component, or all that was necessary for the 

 formation of a new oral apparatus and also for the independent de- 

 termination of a cleavage line, localized increase in kinetosomes. and 

 localized differential elongation of kinetics. That the graft might have 

 lacked many or all of the gross structures of the supernumerary oral 

 apparatus and its associated regions is suggested (but not proved I by 

 two facts. First, one product of the first fission lacked them. Second, 

 in many other ciliates new oral structures arise far from the preexist- 

 ing ones, indicating a less obvious mechanism of production than one 

 depending upon proximity to the preexisting oral structures them- 

 selves. Perhaps something else normally associated with, but separable 

 from, the oral apparatus in Paramecium is the basic determiner of 

 formation of vestibule, mouth, and gullet. If so, this — or part of it- 

 hut not the oral structures themselves, may have been in the active 

 grafted piece; in which case, the gross structures would have arisen 

 de novo, i.e., indirectly, under the influence of a particular, different 

 local cortical pattern. 



Such a mode of origin is certainly involved in the formation of the 

 cytopyge. As pointed out, it arises at the juxtaposition of the posterior 



