Role of Preformed Structure in Cell Hen-din 197 



fields abut and possibly induction of vestibule, moutb, and gullet in 

 a comparable way. 



In all these normally correlated events, only a few major types of 

 processes seem to be taking place: localized increase in the number of 

 kinetosomes; localized elongation of kineties in a definite space-time 

 pattern; and progressive differentiation of particular new forma- 

 tions into definite structures. That there is a decisive developmental 

 and genetic role of preformed structure in these processes is evident 

 from the correlations, which have repeatedly been pointed out, be- 

 tween supernumerary and reduced cortical parts and the numbers 

 of new parts that arise during fission. Between the old and the newly 

 formed parts there is no evidence of a simple, direct, template-like, 

 causal relationship. Reproduction of cortical structures is typically 

 very indirect, involving a complex, dynamic series of events. But these 

 events are normally correlated in space with the locus of visible cor- 

 tical differentiations. Usually, when the localized patterns are present, 

 even in supernumerary and abnormal positions, the corresponding 

 developmental events occur; when they are lacking, the events fail to 

 occur. Remarkable is the fact that such localization of origin and de- 

 velopment of parts and even of differential growth occurs within the 

 confines of a single cell. 



The major remaining problems are to identify the nature of the 

 particular cortical patterns correlated with and presumably deter- 

 mining the particular pattern changes that lead to the development of 

 new structures, and to discover the nature of the interactions that 

 appear to occur between diverse local regions. Whatever these may 

 be, the end result is two from one, two new cortical patterns normally 

 identical with and essentially dependent upon the original one, both 

 when the original is a typical singlet and when it deviates in any one 

 of several ways from the typical singlet organization. In other words, 

 the processes we have been describing at the gross level of microscopic 

 visibility are the processes underlying cell heredity of the cortex and 

 its highly differentiated parts in Paramecium aurelia. 



IV. Literature. Discussion, and Conclusion 



Are the conclusions reached from our study of one stock of one 

 species of ciliate of limited or general applicability? How should they 

 be modified, if at all, in the light of related work and thought on other 

 organisms? These questions would of course be appropriate in regard 

 to conclusions based on the study of any one organism, perhaps espe- 

 cially when that organism is unicellular and a ciliate, for these organ- 



