DIFFERENTTATTOX IN CTLTATES 75 



has to be formed anew from the kiiiety x in each tomite. 

 And the mouth of many cihates behaves in the same way. 

 Differentiated organelles are very often unable to divide. 



According to Tartar, ^'Ciliates show in a dramatic way 

 that cell division is not incompatible with differentiation. 

 The division period is in fact just the time of greatest for- 

 mation of new structures and the two processes of morpho- 

 genesis and division run parallel in time." 



Of course, if differentiation is by definition considered 

 incompatible with cell division, ciliates are not differenti- 

 ated organisms. But the interval of time between two divi- 

 sions of Euplotes is 24 hours, and the morphogenetic proc- 

 esses start 8 hours before the cytoplasmic cleavage. As a 

 matter of fact, "division" is a very complicated process. 

 Two new^ "morphogenetic fields" are formed which organize 

 the proter and the opisthe in the anterior and posterior 

 parts of the parent. The differentiated structures of the 

 parent then disappear. "Division" stricto sensu separates 

 two highly differentiated ciliates. The important phenome- 

 non and the great mystery in this division are the formation 

 a long time before nuclear division of two "morphogenetic 

 fields." It is quite obvious that it is not a "differentiated" 

 ciliate which "divides" but that cleavage separates two al- 

 ready highly differentiated ciliates. The real division has 

 taken place 8 hours before, at the time where two new mor- 

 phogenetic fields have appeared. Although the old. differ- 

 entiated, parental structure is still there at this period, it is 

 nevertheless a dead structure, sentenced to dissolution and 

 disappearance. It is therefore clear that the term "differen- 

 tiation," considered as an irreversible modification of a cell, 

 cannot be applied without discussion to a non-cellular or- 

 ganism. 



Differentiation in ciliates is synonymous with changes 

 occurring during morphogenesis or development. But this 

 organization, when considered independently of the phase 

 of the life cycle, is reversible. It is reversible because it rep- 



