64 BIOLOGY OF DEATH 



years. There are on record instances of fowls living to 

 as many as 20 years of age. But these are wholly excep- 

 tional instances, unquestionably far rarer than the occur- 

 rence of centenarians among human beings. There can 

 be no question that the nine years of life of Carrel's 

 culture has removed whatever validity may have origin- 

 ally inhered in Harrison's point. And further the cul- 

 ture is just as vigorous in its grqwth today as it ever 

 was, and gives every indication of being able to go on 

 indefinitely, for 20 or 40, or any desired number of years. 

 The potential immortality of somatic cells has been 

 logically just as fully demonstrated in another way as 

 it has by these tissue cultures. Some nineteen years ago, 

 Leo Loeb first announced the important discovery that 

 potential immortality of somatic cells could be demon- 

 strated through tumor transplantations. His latest sum- 

 mary of this work may well be quoted here : 



"We must remember that common, transplantable tumors are the direct 

 descendants of ordinary tissue cells, such as we normally find in the 

 individuals of the particular species which we use. The tumors may be 

 derived from a variety of normal tissues and, in general, the transfor- 

 mation from normal cells into tumor cells takes place under the influence 

 of a long continued action of various factors enhancing growth. Tumor cells 

 are, therefore, merely somatic cells which have gained an increased growth 

 energy and at the same time somehow gained, in some cases, the power to 

 escape the destructive consequences of homoiotoxins. This ability of cer- 

 tain tumors to grow in other individuals of the same species has enabled 

 us to prove, through apparently endless propagation of these tumor cells 

 in other individuals, that ordinary somatic cells possess potential im- 

 mortality in the same sense in which protozoa and germ cells possess 

 immortality. Thus tumor transplantation made possible the establishment 

 of a fact of great biological interest, which, because of the homoiosensitive- 

 ness of normal tissues, could not be shown in the latter. 



"We wish, however, especially to emphasize the fact that our experi- 

 ments did not merely prove the immortality of tumor cells, but of the 

 ordinary tissue cells as well, the large majority or all of which can be 

 transformed into tumor cells. At an early stage of our investigations 



