710 



REPRODUCTION AND GROWTH 



From these and many other observations it follows that the physical devel- 

 opment of girls is run through in less time than that of boys and that it 

 terminates more abruptly. 



Hand in hand with the more rapid development in height and weight dur- 

 ing the puberty period, there goes also a correspondingly stronger development 

 of the chest, as contrasted with the development during the years just preceding 

 puberty. The mean increase in the chest measurement at the position of deepest 

 inspiration, in boys from ten to seventeen years of age, is given by Kotelmann 

 (for Hamburg) as follows: 1.68 cm., 1.97, 1.82, 0.99, 3.78, 3.47, 4.02, 2.44 cm. 

 respectively. 



According to Key's observations, the resistance of the body to harmful influ- 

 ences during the years just previous to puberty is relatively weak. But in the 

 course of the puberty period, when the youthful life asserts itself in all its vigor, 



1,50 it 



8 1,60 X 



6 8 1,70 



6 8 1,80 2, <t 6 8 1 ( 90 



FIG. 305. Variations in the height of military recruits in Sweden, after Hultkranz. The ordi- 

 nates represent the percentage of the whole number (232,367) of individuals who were of the 

 height (in meters) represented by the abscissae. 



the resistance of the body increases, the percentage of sickness in the schools 

 declines and reaches its minimum in the last year of this period (seventeen). 



The economic circumstances in which the children live exercises a very con- 

 siderable influence on their physical development. The children of the poorer 

 classes fall behind their companions of the same age from the homes of the 

 well-to-do, both in height and weight. The period of feeble development just 

 before puberty is longer for the poorer children. But once puberty begins, the 

 period of rapid growth seems to proceed all the more rapidly in spite of its 

 delay, and to terminate in the same year as for children of the well-to-do. The 

 whole period is therefore shorter for poor children, but is characterized by an 

 even more active development during its last years (Key). 



Malling-Hanseii has shown by a very extensive series of observations in Den- 

 mark that there is a seasonal variation in the rate of growth in children. From 

 the end of November or the beginning of December to the end of March or the 

 middle of April children grow very feebly so much so that the increase in 

 height is less than usual, however slow that may be. After this period of feeble 

 growth follows a period during which growth in height is very active, but the 

 increase in weight is reduced to a minimum. Indeed, children even lose in 

 weight during this period of most active growth in height, almost as much as 

 they have gained in the preceding period. This period lasts from March or April 



