230 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



all instances, the fibril is heteropolar and could have some signifi- 

 cance in determining spatial relationships. Ciliates have carried 

 to an impressive extreme the elaboration of tubular-fibrillar 

 structures, with and without evident kinetosome attachments; 

 these include orderly arrangements of pellicular fibrils reminiscent 

 of the euglenoid-bodonid type. 



In both the zooflagellates and the ciliates, the evolutionary trend 

 appears to have been one toward increasing architectural com- 

 plexity primarily involving fibrillar structures. It is at least 

 interesting to point out that the straight, orderly, tubular fibril 

 has not, to the author's knowledge, been described from any 

 metazoan cell, except in cilia, kinetosomes, and centrioles. 

 Metazoa have fibrillar structures — spindle fibers, muscle 

 filaments, tonofilaments, neurofilaments, ciliary roots, and con- 

 nective tissue components. Of these only the spindle fibers and 

 ciliary apparatus are kinetosome-linked, and only the highly 

 specialized striated muscle filaments are packed with the con- 

 spicuous precision characteristic of so many protozoan organelles. 

 Metazoa have gone in for membrane elaborations and these by 

 their very nature cannot assume the extraordinary geometric 

 variety that is possible for fibers. No metazoan cell approaches in 

 architectural complexity the design of the "simplest" ciliate or 

 metamonad. 



One gains the impression that the ancestors of zooflagellates 

 and ciliates (whether or not these were the same, and whether or 

 not the euglenoid type was involved) committed themselves (and 

 their kinetosomes) to creative fibrillogenesis, and having done so 

 shortly passed a point of no return. Fibrils could be made to 

 answer many special requirements of an independent cell and did 

 so with outstanding success. But there is no evidence from 

 electron microscopy to suggest that this led to evolution beyond 

 the protozoan level. 



Perpetuation of an extremely complex and individualistic cell 

 pattern must place severe demands on the cell's genetic mechanism. 

 Perhaps the elaborate centriolar structures of the metamonads and 

 the heterokaryote condition of ciliates are devices to assist in 

 meeting these demands. It is a commonplace of metazoan 

 morphogenesis that cells with a high degree of morphologic 

 differentiation are not capable of continued division. But ciliates 



