Role of Preformed Structure in Cell Heredity 201 



with each other and with singlets of the same clone or clones. The 

 point is even more striking in Stentor douhlets: they can he produced 

 hy experimentally relocating parts of one cell (Tartar, 1961; Uhlig, 

 1960 I . 



The production of doublets from parts of one cell or hy fission or 

 conjugant fusion of two identical cells, the origin of subclones of 

 singlets from douhlets, and the limited breeding results from crosses 

 of singlets by doublets, all argued strongly for the absence of pertinent 

 nuclear differences between the two types of cells before our present 

 study proved the point beyond reasonable doubt by exhaustive anal- 

 ysis. 



The role of the cortex and its parts in determining the hereditary 

 difference between singlets and doublets, as well as in determining the 

 production and ordering of the diverse localized cortical parts, has 

 also long been studied in many ciliates I Faure-Fremiet, 1945-19541. 

 Most of these studies are based upon, stimulated by, or related to the 

 kinetosome theory, which can therefore serve as a point of departure 

 for the following discussion. 



To Lwoff (1950) we are indebted for a succinct, documented, 

 thoughtful, and influential account of the kinetosome theory. This 

 theory is concerned with the genetic and morphogenetic properties of 

 kinetosomes and kinetics. The basic genetic feature of the theory is its 

 attribution of genetic continuity to the kinetosomes. Kinetosomes are 

 held to arise only by division of preexisting kinetosomes. The genetic 

 continuity of kineties is held to he due to the retention of products of 

 kinetosome division in the same row. We shall consider the morpho- 

 genetic features of the kinetosome theory after discussing the genetic 

 feature. 



The genetic continuity of kinetosomes is accepted by some modern 

 workers, rejected hy others. Mazia ( 1961 1 argues, as well as can be 

 argued from what is now known, that kinetosomes reproduce them- 

 selves from a germinal part. His evidence is largely derived from 

 studies of centrioles; but centrioles are kinetosomes, according to the 

 kinetosome theory, and this is supported by their similar fine struc- 

 ture (De Harven and Bernhard, 1956). On the other hand. Ehret and 

 Powers (1959) contend that definite proof of kinetosomal self-repro- 

 duction by division is lacking. The mere appearance of a new kineto- 

 some beside an old one is not proof; and no one has yet demonstrated 

 in a new kinetosome material contributed by a preexisting kinetosome. 

 Ehret and Powers suspect that minute unit elements formed deep in 

 the cell (in Paramecium) migrate to the surface and there grow 



