206 The Nature of Biological Diversity 



Although operative analyses of the sort carried out on Stentor have 

 not heen performed on Paramecium, there is nevertheless much to 

 indicate that fundamentally the same models may apply, as will now 

 he set forth. 



The existence of cortical gradients in Paramecium and morpholog- 

 ical evidences of their direction are shown hy three lines of evidence. 

 First, the position of lateral spines, a type of abnormality studied by 

 Jennings (1908) , shifts with the growth and division of the paramecia. 

 The nearer the spines are to the ends of the body, the less their posi- 

 tion shifts per fission cycle; the greatest shifts occur when a spine is 

 near the equator. This indicates that growth is greatest in the equa- 

 torial zone and decreases toward the poles. Second, slow growth at the 

 poles is further indicated by the relatively slight restoration of cut 

 ends at a single fission; several fissions are required to complete re- 

 generation of polar parts (Tartar, 1954b and others). Third, the 

 greatest increase of kinetosome number during fission occurs near 

 the equator; there is less and less with increasing distance from the 

 equator. Thus all three lines of evidence point to a growth gradient 

 which is high near the equator and which decreases toward both 

 poles. 



Paramecium, like Stentor, also possesses sharp juxtapositions be- 

 tween areas exhibiting different kinety patterns. The oral meridian, 

 virtually from pole to pole, is a line of juxtaposition between mark- 

 edly diverse kinety fields on its right and left (page 169). There may 

 even be differences on the two sides of the line in distance between 

 rows of kinetosomes, those on the left usually being greater than those 

 on the right. But this difference, if extensive measurements confirm it, 

 is much less than the corresponding difference in Stentor. Much more 

 striking is the difference on the two sides of the mouth. The vestibular 

 kineties are very closely packed and run parallel to the rim of the 

 mouth; the adjacent kineties forming the gullet not only are differ- 

 ently spaced ( some closer, some further apart ) but are mostly oriented 

 in a different direction. Ehret and Powers (1959) point out two other 

 differences on the two sides of the vestibule-gullet juncture: hexag- 

 onal versus rhomboidal patterns of their "ciliary corpuscles" and 

 one or two versus four kinetosomes per repeating unit. 



The important question is whether one or more of these abrupt 

 junctures of visibly diverse cortical patterns has morphogenetic sig- 

 nificance. The postoral juncture on the two sides of the cytopyge 

 clearly does, as we have shown (page 1841. It will be recalled that 

 the morphogenetic action at the postoral juncture is independent of 

 right-left relations, as was also found to be true for primordium 



