CONDITIONS OF CELLULAR IMMORTALITY 73 



at once puts a new face upon the whole matter. Dawson 

 has studied in continued culture one of these amicronu- 

 cleate races of Oxytricha hymenostoma Stokes. His con- 

 clusion is as follows : 



"The existence of a form which not only apparently may live indefi- 

 nitely without conjugation, autogamy, or endomixis (assuming the possi- 

 bility of the latter phenomenon in an hypotrichous form), but also 

 apparently does not possess the ability to undergo any of these phenomena, 

 brings to light an entirely new possibility in the life history of ciliates. 

 It has been proved quite conclusively, (Woodruff, '14), that in forms 

 which ordinarily conjugate, the continued prevention of this process brings 

 about no loss of viability if a favorable environment be provided. How- 

 ever, in the organism under consideration there is apparently no possi- 

 bility not only of conjugation or endomixis, but also of autogamy; and 

 thus we have from another source crucial evidence that none of these 

 phenomena is an indispensable factor in the life-history of this hypo- 

 trichous form." 



In the light of these clean cut and definite results 

 one is more disposed than was formerly the case to 

 accept at their face value the results of Enriques with 

 Glaucoma pyriformis, and those of Hartmann with 

 Eudorina elegans, in which reproduction went on indef- 

 initely with undiminished vigor and no evidence of any 

 process comparable to endomixis. 



Altogether, it seems to me that the weight of the evi- 

 dence now is that in the Protozoa, senescence (or death), 

 is not a necessar'i^ or inevitable consequence of life. 

 Given the appropriate and necessary conditions of envi- 

 ronment, true immortality — the absence of both senes- 

 cence and natural death, each defined in the most critical 

 manner — ^is in fact the reality for a number of forms. 



Turning to the metazoan. side of the case, the evidence 

 regarding senescence is equally cogent. In the first place, 

 in the longest continued in vitro tissue cultures known 

 (those of Carrel) there is, as already stated, no appear- 



