146 GROWTH 



volved in development, as was presented in detail by Professor 

 Brody in the second lecture of this series, he arrives at certain 

 mathematical curves and graphs expressing the relations be- 

 tween these factors and the time of the life cycle. For example, 

 in the rat, in the ox, in the human, the growth curve is charac- 

 teristic and in most respects very similar. It begins at a certain 

 rate of acceleration, passes through the major part of a growing 

 cycle at a definite rate, and as age approaches that rate slows 

 down to a level of no addition to the mass, that is, no growth. 

 Growth in this sense is a function of time and mass. Height or 

 breadth, or any linear dimension can be taken and by applica- 

 tion of the formulae the growth curves can be constructed. 



Another point of view was emphasized by Dr. Jackson, 

 namely, that growth in mass of the whole animal is the sum of 

 the growth of the parts. He gave evidence to show that the rate 

 of growth in mass of the individual parts of the animal body, 

 when expressed in the form of curves, shows phases of accelera- 

 tion and of retardation of growth at different periods in the life 

 cycle. The periods of maximal growth do not coincide for the 

 different organs. There is in the anatomical museum at Mc- 

 Alester Hall a series of models of human embryos of different 

 ages constructed by Dr. Jackson and his students, in which the 

 body trunk of each embryo is reconstructed on a scale of mag- 

 nification to enlarge it to the length equivalent to that of an 

 adult. These models are a striking illustration of the fact that 

 the relative size and rate of growth of the vital organs varies 

 extremely at different ages. In the very early embryo the heart 

 and the brain are relatively very much larger than they are in 

 the adult. Examining a series of models representing as many 

 ages one cannot escape the conclusion that the heart grows ex- 

 tremely rapidly in the early embryo. Later, as the size of the 

 body as a whole becomes larger, the heart is ever relatively 

 smaller and smaller. Of course the heart actually continues to 

 grow in volume, but at a very much reduced rate. A similar com- 

 parison may be made with reference to the brain. The brain 



