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The Nature of Biological Diversity 



singlets and doublets. However, so potentially important a conclusion 

 should obviously not be left to stand upon mere apparent exclusion 

 of alternatives; it requires the strongest of direct evidence. Such direct 

 evidence might be sought by investigating the genetic consequences of 

 adding and removing cortical parts alone. Would grafted extra cor- 



II 



in 



IV 



FIG. 6. Diagram showing the cellular determination of nuclear size independently 

 of nuclear origin, a, b, d, and e are the same as in Fig. 5; solid macronuclei de- 

 rived from macronuclei of doublets; hollow macronuclei derived from macronuclei 

 of singlets. I and IV are adult cells before and after a fission, respectively. II shows 

 fission with unequal macronuclear division, an effect of homozygosis for the re- 

 cessive gene, am. Ill represents the fission products that received much less than 

 half of the dividing macronucleus. The small piece of macronucleus, regardless of 

 its origin (see Fig. 5) from singlets (hollow) or doublets (solid), grows into a 

 normal-sized macronucleus in singlets and into an oversized macronucleus in 

 doublets. 



