444 Gary N. Calkins. 



tion and involves a factor which cannot be overlooked in any dis- 

 cussion on old age. 



In the June period, as stated previously, the same conditions 

 w^ere not observed, for the organisms, in part at least, had been 

 treated with the beef extract every week during the first three 

 months, since the previous period of depression. The division 

 rate began to run down in the case of the B series in April, in the 

 A series in May, and in all of thq material that had been continued 

 on the beef, the characteristic structure was a densely granular 

 endoplasm (Fig. 7). In the specimens that had not been treated 

 with the beef since the preceding December, this character of the 

 endoplasm was not noted. These unstimulated individuals died 

 out in about the 508th generation (B series) after becoming much 

 emaciated and reduced in size, and with reduced nuclei. The 

 nature of the protoplasmic changes is indicated, in one case at 

 least, in Fig. 14. Here the macronucleus has entirely disap- 

 peared, not even a granular trace remaining, while the endoplasm 

 is crowded with vacuoles of considerable size. The micronucleus 

 is slightly hypertrophied and has a very peculiar outer membrane 

 within which the chromatin and achromatic material lie in what 

 appears to be the real nuclear membrane. The dense granules 

 characteristic of the beef-fed individuals are absent. The un- 

 stimulated A series did not die out until about two weeks later. 

 At the time when the B individual described above died (May 12) 

 the unstimulated A series was characterized by somewhat reduced 

 size, a declining division rate, and absence of the dense protoplas- 

 mic granules. In the stimulated A series, on the other hand, 

 (Ai and A2) of about the 560th generation, the structures were 

 normal, gastric vacuoles were numerous and divisions were fre- 

 quent. Towards the end of June, however, when the A series 

 nearly died out in the 620th generation, the conditions were very 

 different. Fig. 7 is from a specimen in the 615th generation. Its 

 size is below the normal; its endoplasm is choked up with granules 

 and there is no trace of vacuoles save the contractile vacuole near 

 one end. The macronucleus is definitely granular, and its con- 

 tour 'is irregular as though devoid of nuclear membrane. The 

 micronucleus is elongate and spindle-formed. The ectoplasm is 



