Role of Preformed Structure in Cell Heredity 171 



used here, cortex is synonymous with ectoplasm. Some authors use 

 the term "pellicle'' in the same sense; others use it only for the outer- 

 most part of the cortex.) For present purposes, we need only to ex- 

 plain how the orientation of a kinety is recognizable. It is defined hy 

 several features. In the first place, trichocysts alternate with one or 

 two kinetosomes in the anteroposterior direction along a kinety. Sec- 

 ond, a kinetodesmal fiber passes from the kinetosome to the right and 

 anteriorly. Third, on the right side of each kinetosome (or longitudi- 

 nally disposed pair of kinetosomes) is another structure known as the 

 parasomal sac ( Ehret and Powers, 1959 ) . The kinety, its antero- 

 posterior direction, and its right-left orientation are unambiguously 

 defined by these criteria, as microscopic examination of any short part 

 of a kinety clearly reveals. 



The photographs printed here do not show all of these orienting 

 features. The kinetodesmal fibers never appear. The position of the 

 trichocysts is shown only irregularly because the fainter and smaller 

 dots marking their positions (see Fig. \A at edge) are at a slightly 

 different optical level from the larger, darker dots marking the posi- 

 tions of the kinetosomes. When the latter are in sharp focus, the 

 former are either in poor focus or out of focus entirely. However, the 

 photographs often show a blurred, faint, broken line partially con- 

 necting the dots in the same kinety and thus showing the direction of 

 the kinety. This serves to remove ambiguity in interpreting kinety 

 direction, as for example in the anterior left field on Fig. IB. 



What then are the dots on the photographs? They are silver deposits 

 resulting from the use of a modification of the wet-silver impregna- 

 tion technique of Chatton and Lwoff (1930). Each of the heavier dots 

 marks the position of one or two cilia ( and kinetosomes ) and one 

 parasomal sac. With the phase contrast condenser the dot can be 

 resolved into its two or three components; the parasomal sac is always 

 on the right and the one or two kinetosomes are always on the antero- 

 posterior line of the kinety. As Dippell ( 1962 ) has shown, these dots 

 are deposited on the body surface; they are not within the cortex. 

 They mark the positions where the cilia and trichocysts meet the 

 body surface. 



It. The cortical pattern in doublets 



The structural features of the normal single animal are all present 

 in duplicate in doublet animals (Fig. 3 I. The two sets of structures 

 are in homopolar orientation with 180° separating each pair of homol- 

 ogous structures: the two contractile vacuole meridians, the two oral 



'tv 



