CHAPTER XIX 



MECHANICS OF DEVELOPMENT 



Biologically considered, development includes the growth of 

 the individual from the fertilization of the egg to the time of 

 death. Birth is a poor developmental landmark, for there is a 

 wide variation in the degree of development at the time of birth 

 or hatching. The amphibian hatches as an immature larva, 

 while the reptile is usually highly organized and is anatomically 

 a miniature adult. Similarly, the degree of differentiation in 

 different groups of birds or mammals is extreme. Compared 

 with a newborn calf, a marsupial at birth is an early embryo 

 with a few specializations. And comparing man with his nearest 

 relatives, the anthropoid apes, similar although less pronounced 

 differences exist. 



By carrying developmental processes over into postnatal 

 growth it is found that the races of man grow and develop at 

 different rates of speed. The changes in the individual are, how- 

 ever, continuous. Birth causes a complete shift in environment, 

 with the attendant changes in the anatomy of the nutritive 

 mechanisms, for the individual passes from a parasitic life to 

 one of partial independence. But, although birth is one of the 

 developmental landmarks, there are probably as great physio- 

 logical changes at puberty as at birth. Therefore, a considera- 

 tion of anatomy and the functions of structures necessarily de- 

 pends upon the developmental changes which occur in the 

 individual or the race. 



The regulators of development fall naturally into three 

 groups: (1) the differentiation which exists in the protoplasm 

 of the egg; (2) the organizators which are developed by tissues 

 or organs, and which influence the growth processes of other 

 structures; and (3) the definite organs of internal secretion 

 which develop during embryonic life and perhaps play the 



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