1006 THE PHYSIOLOGY OF REPRODUCTION. 



1000 calories of food 340 calories are applied to growth. In man, 

 on the contrary, the growth quotient is only 5 per cent. This 

 growth quotient is a specific property of the cell and a charac- 

 teristic of youthfulness. It has its maximal value at birth, so 

 far as extra-uterine life is concerned, and then sinks slowly, so 

 that at maturity, that is, at the end of the growth period, it becomes 

 zero. Thence forward the energy of the food is utilized only for 

 the maintenance of the cells and for the work they perform, none 

 is applied to growth. Rubner suggests that the power to grow 

 possessed by the cells of the young organism depends upon some 

 special mechanisms of a chemical nature, that is, probably certain 

 special chemical complexes which are responsible for the " growth 

 tendency" ( Wachstumstrieb) . In connection with this growth 

 energy or growth tendency it will be remembered that in the 

 chapter on Internal Secretion evidence was given that in early 

 infancy the thymus forms apparently an internal secretion or 

 hormone which controls or stimulates the process of growth, and 

 the anterior lobe of the pituitary gland also forms an internal 

 secretion which has a similar action. It will be noted also that 

 both of these glands affect mainly the growth of the skeleton. 

 The increase in size of an animal is normally estimated largely 

 from the growth of the skeleton, and Aron has shown in a most 

 interesting way that the growth energy resides chiefly in this 

 tissue. According to this author, young growing dogs if given 

 a diet insufficient to maintain their body weight will still continue 

 to grow, since the skeleton increases in size at the expense of the 

 other tissues, particularly of the muscular tissues. The growth 

 tendency in the skeletal tissue is so strong that other tissues are 

 absorbed to furnish the necessary material. This marked growth 

 tendency of the skeleton, as we have just said, is controlled or 

 stimulated by secretions from the thymus and hypophysis and 

 possibly from other sources. The fact that a tissue in which the 

 growth tendency is marked will live at the expense of other 

 tissues finds an illustration in other ways, for example, in the 

 development of malignant growths, such as cancer or in the pro- 

 cesses of regeneration in the lower forms of life. Stockhard reports 

 that in the medusa, when unfed, a regenerating tissue may grow 

 rapidly by feeding on the old body tissues. It would seem that 

 this tendency to grow must, as Rubner suggests, depend upon 

 some peculiarity in the chemical structure of the tissue which 

 exhibits it. Moreover, there is considerable evidence that for 

 growth to take place certain specific building materials must be 

 furnished in the diet. As stated on p. 902, Osborne and Mendel 

 have shown that in growing rats certain proteins supply this 

 material; other proteins apparently are lacking in it, but may 



