124 



GROWTH 



depth in Figure 68), according to the general formula: y = ex. 

 This signifies a relationship like that previously- shown for the 

 fish. The thoracic dimensions accordingly remain in constant 

 proportion to body length, during the fetal period. The second 

 group includes head and neck dimensions (exemplified by head 

 length in Figure 68), according to the general formula: y = 

 ex + b. In this case, although each dimension is increasing in 

 proportion to the body length during the fetal period, the posi- 

 tive b indicates a relatively 

 larger size during the preced- 

 ing embryonic period. The 

 third group includes in gen- 

 eral the pelvis and extremities 

 (exemplified by pelvic width 

 and foot length in Figure 68), 

 according to the general 



Figure 68. Graph showing typical external 

 dimensions during the human fetal growth 

 period. Data from Calkins and Scammon. 6 



formula: y = ex — b. In this 

 case, although the dimensions 

 are increasing in proportion to 

 the body length during the 

 fetal period, the negative b indicates a relatively smaller size 

 during the preceding period. 



On first thought it might seem that, since all external dimen- 

 sions are increasing uniformly in relation to body length, the 

 body form should remain constant during the fetal period. This 

 applies only to the thorax, however. On account of their rela- 

 tively greater initial size, the head and neck dimensions are be- 

 coming relatively smaller j while the reverse is true for the 

 pelvis and extremities. 



The changes in proportional growth in various parts of the 

 body axis are also clearly evident in Figure 69, showing the 

 percentage length of various regions in the human vertebral 

 column at different stages of development. The thoracic region 

 remains fairly constant at slightly less than 40 per cent of the 

 column from the early embryonic to the adult stage. The cervi- 



