Biology 697 



fruit-fly was 54 days at ordinary temperatures, it was only 21 

 days at 30 C., but could be prolonged to 177 days by keeping 

 the animals at 10 C. 



But in warm-blooded animals with an adult period in which 

 no growth takes place, such as birds and mammals, the span of 

 life cannot be lengthened in any of these ways. The adult period 

 is one of very carefully adjusted balance, and when the balance 

 is upset old age sets in, to be followed by inevitable death. 



Interesting Experiments 



In this connection one very interesting fact has been dis- 

 covered in recent years, namely, that many of the tissues of 

 which the body is composed are potentially immortal, although 

 the body itself is doomed to death. By careful methods it has 

 been found possible to cultivate in nutritive fluids, outside the 

 body, small pieces taken from a living animal, transplanting them 

 to new portions of fluid every few days. Carrel in New York has 

 cultivated a piece of connective tissue, taken from a chick before 

 hatching, for longer than the full normal lifetime of a hen; and 

 what is most remarkable, the rate of growth and multiplication of 

 the cells composing it did not decrease. We must suppose that the 

 balance and interaction of the different tissues, each checking and 

 counter-checking the other, lead to death, whereas the unchecked 

 multiplication of any one sort of cell, if the right conditions of 

 food and expansion are provided, can continue indefinitely. 



If we wish to prolong the existence of the whole, which is 

 our only practical concern, we must attempt to discover what are 

 the organs involved in maintaining the balance of adult life, and 

 then try to help them as they begin to fail. 



10 

 The Ductless Glands 



Our knowledge is still very scanty, but it seems quite clear 

 that the chief organs concerned are on the one hand the so-called 



