34 MORPHOGENESIS IN CILIATES 



cytoplasmic units which we are compelled to postulate, 

 they are a helpful nuisance. 



It is extremely difficult to understand how, during the 

 development of an organism, cells with an apparently con- 

 stant genome may become differentiated. The concept of 

 the unequal distribution of specific units, or self-reproduc- 

 ing plasmagenes, has been built up to explain cell differen- 

 tiation, and the concept of molecular ecology in order to 

 explain cell and tissue organization. But we have no mor- 

 phological or physiological data which would constitute a 

 proof of the existence of any one of these postulated units. 

 Also, it is really strange that biologists, faced with this 

 problem, should have ignored or even denied the possible 

 existence of cytoplasmic, visible, self-reproducing units. 



Chatton and I, when working out the life cycle of apos- 

 tomatous ciliates in the years 1923-1932, did not realize 

 completely the possible importance of the kinetosomes for 

 some of the future problems of developmental physiology 

 and of molecular ecology. 



The study of apostomes shows that different morphologi- 

 cal types, different structures or patterns, organelles of dif- 

 ferent physiological function may be formed from the very 

 same material, the kinetosome. The condition for these 

 changes is a heterogeneous cortex. The movements of the 

 kinetosomes are controlled by the metabolism. 



A gene mutation modifying the enzymatic system would 

 be able to produce hereditary morphological changes. But 

 it must not be forgotten that kinetosomes represent a popu- 

 lation of "self-reproducing" units. These units, like any 

 other, must be able to undergo changes. If a change is 

 favorable, the "mutated" kinetosome may replace the oth- 

 ers. In many ciliates, kineties reproduce themselves by 

 elongation; they are endowed with genetic continuity. It 

 is therefore possible that different mutants of kineto- 

 somes are selected in different meridians. Nevertheless, 

 movements of kinetosomes are controlled by the metabo- 



