ASPECTS OF THE KINETOSOME 33 



these phenomena is excluded. The fates of the kinetosomes 

 are largely phenotypical expressions of changes in the prop- 

 erties of their environment. 



Thus kinetosomes illustrate in a most spectacular way 

 the notion of morphogenetic field and of organizers. From 

 the study of the life cycle of ciliates, it may be concluded 

 that kinetosomes as a whole play a fundamental role in 

 their morphogenesis. 



But ciliates are non-cellular organisms. Are kinetosomes 

 of any use in understanding the problem of morphogenesis 

 in "higher organisms"? It is possible to consider that cells 

 which do not produce cilia are unable to produce the spe- 

 cific substance which enables the formation of cilia. But 

 it is easy also to visualize kinetosomes being segregated, so 

 that cells which possess kinetosomes will develop cilia, 

 whereas cells devoid of kinetosomes will not. 



It is possible furthermore to consider that, if a kineto- 

 some is changed into a trichocystosome, the potentiality of 

 the host-cell will be changed: it will produce trichocysts 

 instead of producing cilia. 



Thus, the kinetosome provides a model for one possible 

 mechanism of cell differentiation in higher animals. 



A multicellular organism has differentiated cells. A cili- 

 ate has differentiated parts. It seems therefore important 

 to point out that self-reproducing particles may behave in 

 different ways in different parts of a ciliate, just as viruses 

 behave in different ways in different cells of an organism. 



C. Darlington (1939) considers the plasmagenes as "a 

 relic of the naked gene of a remote prechromosomian period, 

 a relic which has been preserved in spite of its being a nui- 

 sance in heredity because it is a necessary cog in the ma- 

 chinery of development." Kinetosomes considered as plas- 

 magenes or as visible specific cytoplasmic units endowed 

 with genetic continuity are perhaps a nuisance. But, in 

 providing a useful model for concepts concerning specific 



