138 



ROBERT BENNETT BEAN 



of one structure may he synchronous with the periods of retardation 

 in the development of another, and the two 7nay be called comple- 

 mentary structures. Each organ or structure has a critical period 

 when it is developing most rapidly, and when it is probably most 

 susceptible to its environment. 



Jackson determined the growth in volume of the parts of the 

 body in relation to the whole from observations on 43 human 

 embryos and fetuses, and other similar material. The time at 

 which the structure attains its greatest relative size is given, 

 but this may or may not be the time of greatest relative accel- 



Diagram 1 Prenatal growth in man. The curved lines represent the month 

 at which the organs named above grow most rapidly. The diagonal lines repre- 

 sent the approximate relative rate of growth of the organs named thereon. 

 The number of the months is written below. 



eration in growth, and it certainly is not its greatest absolute 

 extent of growth, all of which should be considered in any study 

 of growth. 



The growth of any part of the body should be determined in at 

 least three ways. First, the amount of growth in relation to the 

 size of the part previous to the period of growth; second, the 

 amount of growth in relation to the adult size of the part; and 

 third, the amount of growth of the part in relation to the total 

 growth of the individual. The first is important because it 

 would give the real activity of the part at different stages of its 

 development, yet this method is not usually adopted. Diagrams 

 1 and 2 will serve, however, to illustrate the relative growth of 

 the body parts. 



