Role of Preformed Structure in Cell Heredity 179 



clones and those from doublet conjugants (/) produced doublet clones. 

 This evidence thus supports that in the preceding paragraph which 

 excluded macronuclear differentiation as the basis of the singlet- 

 doublet difference. 



Another conceivable nuclear basis for the singlet-doublet difference 

 lies in nuclear size. Without making measurements, it is obvious that 

 doublets carry larger macronuclei than singlets. Do the larger nuclei 

 determine larger cells with more cortical structures? If so, is the size 

 of the nucleus an inherent property of the nuclei? The experiments 

 which tested the hypothesis of macronuclear differentiation provided 

 the answer. Mere inspection, without measurement, showed that when 

 a fragment of macronucleus from a doublet regenerated in a singlet 

 (type b, Fig. 5) it grew into a macronucleus of normal size, i.e., the 

 size characteristic of singlets; and that, when it regenerated in a 

 doublet (type d, Fig. 5 ) , it grew to the large size characteristic of 

 doublets. Conversely, when a macronuclear fragment from a singlet 

 regenerated in a doublet (type e, Fig. 5), it grew into a macronucleus 

 of the size characteristic of doublets; but when it regenerated in 

 singlets (type a, Fig. 5 ) , it grew to the size characteristic of singlets. 

 The size of the macronucleus is not a cause of, but a response to, the 

 size of the cell, as has also been reported in Stentor (Tartar, 1961 ) . 



The same conclusion follows from other results in clones homozy- 

 gous for the am gene. As shown in row II of Fig. 6, this gene often 

 yields very unequal macronuclear division at fission (Sonneborn, 

 1954b; Nobili, 1961a, b) . Some cells receive very small pieces of mac- 

 ronucleus (rows II and III, Fig. 6). These small pieces grow into 

 macronuclei of the size characteristic of the type of cell that bears 

 them, regardless of whether the macronuclei were originally derived 

 from singlets or doublets (row IV, Fig. 6) . The observations described 

 in this and the preceding paragraph thus effectively exclude macro- 

 nuclear size as a determinant of the difference between singlets and 

 doublets. 



I have not been able to imagine any other possible mechanism of 

 nuclear control of the difference between singlets and doublets. 

 Therefore, attention was redirected back to possible cytoplasmic de- 

 termination. The test that had been applied ( page 174) was one which 

 could reveal an effect of the freely moving cytoplasm, the fluid cndo- 

 plasm, alone. It could not test an effect of the rigid, nonmobile 1 to 2 

 micron thick outer layer of ectoplasm, the cortex of the cell, contain- 

 ing nearly all the structures which distinguish singlets from doublets. 

 As the only obvious remaining untested part of the cell, the cortex 

 seems to be or to contain the genetic basis of the difference between 



