INHERITANCE 



765 



size obviously depends on what nuclear (chromosomal) combination 

 is present in the two parents after conjugation. 



Repeated crosses between the same two clones give different final re- 

 sults in different cases, just as the same pair of parents in multicellular 

 organisms produce in different cases offspring that differ in hereditary 

 constitution. Table 20 shows the different final sizes reached by the de- 

 scendants in repeated examples of a number of different crosses. Figure 



Table 20: Lengths in Microns of the Two Races Crossed (Desig- 

 nated A and B in Each Case) with the Resulting Final Lengths 

 of the Offspring, Paramecium caudatum; in Cases in which Several 

 Different Pairs Were Obtained from the Same Cross, Their De- 

 scendants Are Numbered 1, 2, 3, 4 (Data from De Garis, 1935) 



178, drawn to scale, shows graphically the relative sizes of descendants 

 of different pairs in certain crosses. 



The fact that different pairs of conjugants from the crossing of the 

 same two clones give descendants of different sizes indicates that in these 

 Protozoa, as in Metazoa, recombination of the chromosomal materials 

 occurs, giving different combinations in different cases. It shows also 

 that clones of P. caudatum must in many cases be heterozygotic for size 

 factors; otherwise different results would not be produced from different 

 pairs of the same cross. 



The fact that the descendants of the two ex-con jugants of any pair 

 yield descendants that are finally of sensibly the same size shows that 



