INHERITANCE 763 



The large ex-conjugant A now divides by fission. Its two offspring 

 grow to the large size usual for race A. The small ex-conjugant B di- 

 vides; its two offspring grow only to the small size usual for race B. The 

 offspring divide again, and fission continues at the rate of once or twice 

 a day, each ex-conjugant producing a clone. 



As fission continues day after day, it is found that the adult sizes are 

 changing in each clone. In the clone descended from A, the individuals 

 of the successive generations grow smaller; in the clone descended from 



Figure 177. Changes in mean size of 

 the descendants of the two members, A 

 and B, of an unequal pair, in another 

 cross. At the upper left are shown the 

 relative sizes immediately after conju- A B 4 



gation. Reading from left to right are 

 shown in the three rows (from above 

 downward) the successive sizes of the 

 descendants of the two at intervals of 

 two days, till at the end of twenty-four 

 days (lower right) the two have reached 

 a common small size that is not greatly 

 different from the original size of B. 

 (Based on the measurements of De 

 Garis, 1935.) 



A B -4 ^ ® 



li II II II 



B, they grow successively larger (Fig. 176). The average sizes of the 

 two races approach one another. This continues for twenty-two days, 

 including about the same number of generations. By that time the indi- 

 viduals in both clones have reached a size that is approximately midway 

 between the original size of race A and that of race B. At that point, 

 the changes in size cease; this intermediate size remains constant in the 

 two clones until there is another conjugation. The two clones having 

 come to a common size, now form a single race of uniform adult size. 

 Thus for a long period, twenty-two days, in this case, the size in the 

 descendants is affected both by the cytoplasm and by the nucleus, but 

 finally the size is controlled entirely by the nuclear constitution. During 

 the intervening period the two clones differ in size, and this can be 

 due only to the difference in their cytoplasm, since they are alike in their 

 nuclei. The large cytoplasmic body of A is reduced only slowly and 

 gradually to the new size, and while this is occurring, potentially mil- 



