FORM AND GROWTH 



125 



Embryo Embryo Embryo 



r Jfd&J 



Coccygeal 



Humah Vcrtcbkal Column -Tbvenrdf? IpnpA (fftflcflj 6 



Figure do. Histogram illustrating the 

 changes in the relative length of the vari- 

 ous regions of the human spinal cord at 

 different stages — embryonic, fetal, newborn, 

 and adult. Data from His" and Jackson. 21 



cal region decreases nearly half in relative length during the 

 same period. This is counterbalanced by a relative increase in the 

 lumbar and sacral regions. 

 The coccygeal (caudal) region 

 is variable, but in general de- 

 creases in relative size, in ac- 

 cordance with the rudimentary 

 development of the tail in 

 man. Calkins 5 found that the 

 characteristic regional differ- 

 ences in growth of the verte- 

 bral column apply likewise to 

 the growth of individual ver- 

 tebrae within each region. 



Scammon 6 has demon- 

 strated similar progressive 



changes in the weights of the head, trunk, and extremities, thus 

 confirming the law of developmental direction for growth of 

 these parts during the human fetal period. 



IV. Relative Growth of Systems and Organs 



Having noted the relative growth rates for the chief divi- 

 sions of the body, we may next observe the growth of the prin- 

 cipal constituent organ systems. Some data on the relative 

 weight of the chief systems in the newborn and adult stages of 

 a few typical vertebrates are given in Table I. The skin of the 

 dogfish decreases from about 1 1 per cent of the body weight at 

 birth to 7 per cent in the adult. In the chick, guinea pig, rat, and 

 man the relative weight of the skin in the newborn appears 

 nearly the same as in the adult (including the subcutaneous tis- 

 sue in man). However, this does not necessarily mean that the 

 skin remains in constant proportion to the whole body through- 

 out the entire postnatal period. In the rat, for example, al- 

 though not shown in the table, the skin grows rapidly in the 

 first week after birth, reaching a temporary maximum of nearly 



