138 GROWTH 



tinued throughout life by the wandering cells of the blood. 

 This process is distinct from that in which whole organs are 

 mechanically displaced, due to unequal growth in neighboring 

 regions, as before mentioned for the heart and liver. 



4. Cell she. In general, the cells remain relatively small so 

 long as cell division continues. After cells have ceased to divide, 

 they usually undergo more or less enlargement or hypertrophy 

 during the growth period. Levi and others, however, have 

 noted that large animals differ in size from small animals 

 chiefly in the total number of cells in the body rather than in 

 size of cells. Aside from cells of the permanent type (especially 

 nerve and muscle cells), there is relatively little difference in 

 the size of individual cells between the mouse and the elephant. 



5. Cell differentiation. While the embryonic cells continue di- 

 viding, they remain in general not only small but also relatively 

 poor in cytoplasm and simple in structure. The succeeding pe- 

 riod of cell enlargement is also accompanied by a specialization 

 in structure and function, with a loss of capacity for cell divi- 

 sion. This rule holds in general throughout the body, although 

 the time and manner of the cell specialization vary greatly 

 among the constituent parts, organs, tissues, and cells. Minot 

 described the life history or cytomorphosis of every cell as a 

 cycle with various stages. Each cell passes through an early un- 

 differentiated stage, followed by a stage of differentiation and 

 final stages of death and removal. But differentiation begins 

 much earlier in some cells than in others, and the length of the 

 life cycle also varies from a few days in some cells to many 

 years in others. Cells in all stages of cytomorphosis can there- 

 fore be found in the body at all ages, although the proportion 

 of undifferentiated cells decreases progressively during devel- 

 opment. In the adult organism, relatively few undifferentiated 

 cells remain, forming the labile type before mentioned. 



We may therefore distinguish, for the body in general as 

 well as in the individual organs, three periods of growth. First 

 comes the embryonic period of growth by numerical increase 



