Role of Preformed Structure in Cell Heredity 213 



important point is whether the evolutionary changes in the eortex 

 were due to genomic changes, independent cortical changes, or paral- 

 lel series of independent but selectively correlated changes in genome 

 and cortex. No direct evidence on these alternatives is availahle or is 

 likely to he obtained. If cortical changes other than mere losses or 

 additions of parts ( for example, shifting the position of the oral appa- 

 ratus from equator to pole, a type of difference that distinguishes 

 certain ciliate taxa ) could he produced and shown to be autonomously 

 inherited, then independent cortical evolution woidd he indicated. 

 Thus far, nothing like this has been accomplished in the laboratory 

 or seems likely to be. I am inclined to favor the possibility of parallel, 

 independent, and selectively correlated evolution of genome and 

 cortex, for it has well-established precedent in the parallel evolution 

 of genome and plastids in Oenothera (Stubbe, 1960; Cleland, 1962). 

 Some of the main trends in cortical evolution are both obvious and 

 instructive. Zooflagellates, from which the ciliates evolved, are an 

 enormously varied group. The progression from simplest to most com- 

 plex is outstandingly accompanied by corticalization of an increasingly 

 large number and variety of kinetosomes and their associated struc- 

 tures. In the simplest flagellates, these are unitary, simple, and for 

 the most part deep in the endoplasm. The centriole is in them the key 

 kinetosome. While it may be compound in structure, it is associated 

 with both spindle fibers and the one or few flagella. In somewhat less 

 simple flagellates, there are one or few kinetosomes and more complex 

 and more numerous fibrous structures, such as cresta, axostyle, and 

 costa, in addition to spindle fibers and flagella. The kinetosomes, as 

 well as most of the other parts, still lie deep in the endoplasm. In the 

 more complex flagellates, several changes occur: the number of kinet- 

 osomes increases greatly, they concentrate more and more upon asso- 

 ciation with flagella, and they lie prevailingly near the surface of the 

 body. Finally, in the opalinids the cortex of the anterior pole or apex 

 of the cell reaches its fullest development as a sort of organization 

 center, with a ring of apical kinetosomes organizing longitudinal 

 kineties. In the ciliates these longitudinal kinetics become independ- 

 ent of the apical region and the cortex of the equatorial region be- 

 comes the center of growth and organization. 



The original internal location of kinetosomes and other parts in 

 Zooflagellates and their evolutionary migration to the cortex support 

 Ehret and Powers' ( 1959 ) view of the internal origin of kinetosomes 

 (or of ciliary corpuscles) in the ontogeny of Paramecium. But other 

 aspects of the evolutionary picture do not support their view that the 

 . cortex of Paramecium is composed solely of packed ciliary corpuscles, 



