io GARY N. CALKINS 



EXPLANATION OF PLATE i. 



M = macronu deus . 

 m=micronucleus. 



FIG. i. Ex-conjugant of the A series in the third generation after union. Four 

 micronuclei can be seen to the right of the macronucleus, and, in addition, a rela- 

 tively large unabsorbed fragment of the old macronucleus. The micronuclei are 

 precociously divided for the fourth generation. 



FIG. 2. Paramecium aurelia in the 24th generation. The two micronuclei 

 may be seen at the lower end of the macronucleus, one on each side. 



FIG. 3. Paramecium aurelia in the 43d generation. The two micronuclei 

 are of relatively large size, and are dissociated from the macronucleus at the upper 

 end of the animal (the two objects at the right of the macronucleus are foreign par- 

 ticles deposited on the organism). 



FIG. 4. Paramecium aurelia in the 46th generation. The two micronuclei 

 are at the upper end of the macronucleus, and cannot be seen distinctly. 



FIG. 5. Paramecium aurelia in the caudatum phase at the 43d generation 

 after union. The single micronucleus may be seen at the extreme right end of the 

 nuclear material. It is noticeably larger than the micronuclei of the aurelia phase 

 (compare Fig. 2). This specimen is one of the same lot as that represented by Fig. 3. 



FIG. 6. Paramecium aurelia in the 32d generation with macronucleus and 

 micronuclei in division. Three of the micronuclei are shown with pointed ends; 

 the fourth is out of focus and does not show. 



FIG. 7. Paramecium caudatum phase of P. aurelia, in the 22oth generation. 

 The relatively large micronucleus is dissociated from the macronucleus. This speci- 

 men was killed when the division rate was high (end of July; cf. curve). 



