CHAPTER D 



Division, Absence of Division, 

 Induced Division, and Morphogenesis 



When considering the Hfe cycles of apostomatous ciliates, 

 one would be tempted to conclude that the arrangement of 

 kinetosomes as it is seen in the structure of the tomite is 

 the indirect consequence, or the necessary corollary, of 

 modifications connected with, or controlled by, division. 

 This hypothesis must now be considered in the light of 

 some aberrant life cycles. 



In any one of the species we have examined, it may 

 happen that the trophont takes up only a small amount of 

 food. These undernourished trophonts encyst. They do 

 not divide. However, they undergo metamorphosis and 

 give rise to a tomite. This tomite will encyst on its host. 



Thus the complete series of metamorphosis, the move- 

 ments of kinetodesma and kinetosomes, take place without 

 division. They are obviously and necessarily the conse- 

 quence of changes in the properties of the cytoplasm, which 

 can only be the reflection of the metabolism as controlled 

 by the interaction of hereditary outfit and external factors, 

 including food. 



The study of Phoretophrya will allow discussion of this 

 obvious conclusion. The grown-up trophonts which escape 

 the exoskeleton of Nebalia encyst and undergo detorsion. 

 They may evolve in two different ways. They may follow 

 the normal route: division, formation of a tomite, encyst- 

 ment. The phoront will escape at the molt. But a certain 

 number of encysted ciliates, for some unknown reason, do 

 not undergo division. They are transformed into one huge 



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