FORM AND GROWTH 137 



stable, and labile. In the 'permanent cells, of which nerve cells 

 and striated muscle are examples, cell division (mitosis) usu- 

 ally ceases rather early in prenatal life, and the number of these 

 cells then becomes fixed and constant. In the stable cells, which 

 include those of most organs, division continues for a longer 

 period, sometimes (to a slight extent) even up to the adult 

 stage. The labile cells continue division throughout life, replac- 

 ing losses in those tissues which are constantly worn off or used 

 up, such as the surface epithelia and the blood cells. In these 

 cells, growth is chiefly by division or hyperplasia, but the total 

 number of cells becomes fairly constant as soon as there is an 

 equilibrium between cell production and cell destruction. 



Many observers have noted that cell division (mitosis) is 

 very active in the most rapidly growing embryonic regions. This 

 indicates in these regions a greater cell activity and cell me- 

 tabolism, which has been demonstrated by Child" 8 and his co- 

 workers by means of various physiological tests. Minot 39 used 

 the term "mitotic index" (number of mitoses per thousand 

 cells) as a measure of the intensity of growth in various parts 

 of the embryo at different stages. Milone 37 has recently applied 

 this method in an interesting study of the growth of the spinal 

 cord in bird embryos of different species. Levi 34 and others have 

 noted the curious fact that mitosis in embryonic tissues is not 

 uniform in rate, even in the most actively dividing cells. There 

 seems to be a tendency to rhythm in the process, with alternate 

 periods of relative rest and activity. 



2. Cell form. In some cases, especially in the earlier stages of 

 development, cells may change their form without change in 

 number or size. They may flatten, elongate, or otherwise change 

 in shape. This may result in a regional change in form without 

 increase in total mass, which corresponds to the plastic type of 

 growth (Pfeffer). 



3. Cell position. Cells may also change their position by in- 

 dividual migration. This occurs, for example, in the embryonic 

 development of the sympathetic nervous system, and is con- 



