1016 THE PHYSIOLOGY OF REPRODUCTION. 



sidered require very different times to double their weight, it 

 follows from this law that the shorter the time necessarj^ for this 

 result the more intense will be the metabolism, or, expressed in 

 another way, the rapidity of growth is proportional to the intensity 

 of the metabolic processes. Second, the law of the constant growth 

 quotient. In all the mammals considered, with the exception of 

 man, the same fractional part of the entire food energy is utilized 

 for growth. This fractional portion is designated as the "gro^vth- 

 quotient, " and it averages 34 per cent., that is to say, for every 

 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 f(jr 

 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). 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 mus- 

 cular 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 is con- 

 trolled or stimulated to a certain extent by secretions from the 

 thyroid and hypophysis and possibly from other sources. The 

 fact that a tissue in which the growth tendency is marked will five 

 at the expense of other tissues finds an illustration in other ways, 

 for example, in the development of malignant growths, such as can- 

 cer or in the processes 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 pecuUarity 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. 901, Osborne and Mendel 

 have shown that in growing rats certain proteins supply this 



