GROWTH, AGE, AND DEATH 293 



Disorders produced by the lack of any of these substances are 

 rather promptly relieved by the addition of the proper vitamin to 

 the diet. Vitamin deficiency, however, renders the animal more 

 susceptible to infection by pathogenic bacteria; consequently serious 

 results may flow from deficient diets. Ordinarily, the varied diet of 

 civilized Man provides all necessary vitamins, so long as moderate 

 attention is paid to the constituents. 



But it must not be supposed that the studies of growth have 

 ceased with the discovery of the facts that have just been outlined. 

 The final word has not yet been said, for investigations of growth 

 and of the nature of growth-promoting and growth-inhibiting sub- 

 stances go on continuously. Particular attention is being paid to the 

 chemical nature of certain proteins or protein-like substances that 

 are concerned in growth; also the nature of the vitamins and 

 their action are being subjected to further analysis. Moreover, 

 growth is being studied from the standpoint of the energy required 

 and the manner in which the supply of energy is transformed, par- 

 ticularly during the early period of development of the animal, 

 when growth is most rapid. 



Growth and Differentiation. The property of continuous 

 growth is exhibited by embryonic tissues, so long as differentiation 

 does not occur. This is illustrated in a most striking way by the 

 behavior of tissues that have been removed from the embryo and 

 induced to live in an artificial medium. More than twenty years 

 ago in a prominent research laboratory, a fragment of heart tissue 

 was removed from an embryo chick and placed in a suitable nu- 

 trient solution at the proper temperature. The culture is still con- 

 tinued. Growth and cell divisions take place but there is no dif- 

 ferentiation; it is today still embryonic heart tissue. At intervals of 

 several days fragments are removed and the nutrient solution re- 

 newed. This culture has been carried on continuously; the amount 

 of embryonic heart tissue that might have been formed during the 

 many years of cultivation is tremendous, exceeding by many million 



