1914I Lafayette B. Mendel 171 



inorganic, — the energy features and other nutritlonal details can be 

 studied with some precision. But of what we have called the 

 internal factor in growth — the growth impulse, the tendency to 

 grow, the capacity to grow, " Wachstumstrieb," " Wachstums fähig- 

 keit," " WachstumsmögHchkeit," — the factor which is hereditary in 

 its origin and sets to growth the hmits which nutrition cannot f un- 

 damentally alter, httle further can be said. It may be that the 

 rythm of cell division and its attendant features, which some have 

 identified as the detectable expression of the capacity to grow, is 

 dependent upon the action of " hormones," products of internal 

 secretion and cellular metabolism. If this is true we shall have at 

 our disposal some means of modifying the internal factors of 

 growth. For the present we must content ourselves with the un- 

 satisfying conclusion that to unravel the inner nature of growth is 

 to penetrate the secret of the distinguishing characteristics of living 

 substance. Here hypotheses reign uncontrolled. 



However dominant the röle of the cell in growth may be, the 

 problem of development cannot be investigated solely from the 

 Standpoint of cellular physiology. We may admit the limitations 

 of nutrition in furnishing an adequate explanation of either the 

 " Wachstumstrieb " or the " Wachstumsmöglichkeiten " ; but in any 

 event the food factor in growth is one that is open to experimental 

 study. The greatest hope of advance in the Solution of the obscure 

 questions of growth therefore appears to lie in this direction. 



Along this direction, for example, Osborne and I have found 

 with our coworkers that not all proteins suffice to promote growth. 

 Some of them are apparently adequate to fulfill the needs of both 

 maintenance and growth; others like gliadin satisfy the requirement 

 of maintenance alone ; while such " incomplete " proteins as zein and 

 gelatin are by themselves inadequate in every sense for perfect nutri- 

 tion. The incapacity of some of these proteins unquestionably lies 

 in a lack of certain essential amino-acid units. It must be noted that 

 growth has not been accomplished with any protein lacking the 

 cyclic groups such as are found in tyrosine, Phenylalanine, and 

 tryptophane; and we have lately found that lysine is indispensable 

 for tissue construction. That these " inefficient " proteins are not 

 primarily toxic to the organism is shown by the fact that we have 

 found many of them to be adequate for maintenance in both grow- 



