210 The Nature of Biological Diversity 



Obviously ciliates would long ago have become extinct were there 

 not also reversals of the unidirectional, progressive, degenerative 

 clonal age changes in morphogenetic processes. Return to the starting 

 point is required for survival. This is commonly accomplished in 

 ciliates at the time of the fertilization processes. In Paramecium, for 

 example, conjugation and autogamy are the only occasions on which 

 a preexisting oral apparatus involutes, disappears, and is replaced by 

 a new oral apparatus (Hertwig, 1889; Roque, 1956a; Porter, 1960). 

 The new one arises in the same manner as at fission, but it remains 

 in the cell in which it arises. At the same time, the cilia on part of 

 the ventral surface of the two conjugants are lost and later regen- 

 erated (Hiwatashi, 1955). Replacement of the corresponding kineto- 

 somes (or ciliary corpuscles) and comparable replacements on the 

 rest of the surface have not yet been reported. In view of the initiation 

 of a new life cycle at fertilization and of the evidence for progressive 

 age changes in the cortex, a deliberate and careful search for more 

 extensive renewal of the cortex at the time of fertilization might prove 

 fruitful. In any case, it is clear that at least some cortical changes 

 form not merely a sequence, but a cycle. Similar cyclic returns to the 

 starting point are associated in some ciliates with encystment and with 

 "physiological regeneration," a disappearance of some old organelles, 

 and their regeneration. 



Cyclic return to the starting point is also shown during the asexual 

 reproduction of parasitic ciliates with multiple hosts (Lwoff, 1950). 

 Markedly different cortical patterns are characteristic of the period 

 of association with different hosts. One pattern leads regularly to the 

 next in correlation with a definite and regular sequence of hosts. The 

 sequence of cortical changes and hosts leads back to the starting point. 

 It too is not merely a sequence, but a cycle. 



These observations on the cycles of cortical changes in the course 

 of the lives of multihost, parasitic, asexual ciliates and of free-living 

 sexual ciliates are of great significance for any general theory of the 

 role of the cortex. They show that, in ciliates, cortical morphogenesis 

 and heredity are not — as is commonly supposed — normally limited to 

 the regular bipartition and reconstruction of exactly the same cortical 

 pattern fission after fission, but are progressive — in part, very slowly 

 progressive — sequences of changes forming a closed cycle back to the 

 starting point. This larger view of the cortical events in ciliates pro- 

 vides a model of what would have to be involved in multicellular 

 organisms if the cortex played in them a role of importance com- 

 parable to the role it plays in ciliates. 



