248 EMBRYOLOGY 



constant rate until a critical number of cells is reached, beyond which the 

 cells cannot divide unless these three variables are changed. Actually some 

 growth still occurs at this critical size, because some cells are dying and 

 being replaced, but the over-all growth has stopped. 



Limitation of growth 



The factor of differentiation also limits cell division and growth. Highly 

 differentiated cells, such as nerve cells, do not undergo cell division. It might 

 be pointed out that nerve cells can still grow, because when the nerve fiber 

 is cut it will regenerate. This means that growth is taking place. Substances 

 are being synthesized to form the new nerve fiber, but the nerve cell itself will 

 not divide. 



Growth in the embryo is subject to an aging process. Tissues removed 

 from embryos of different ages and cultured in a nutrient medium show dif- 

 ferent rates of growth. The older the embryo, the slower the growth rate of 

 its tissues in tissue cultures. Adult tissue grows very slowly. Some adult 

 tissues, like nerve tissue, will not grow at all in tissue culture. However, 

 cancers and tumors grow very rapidly both in the adult or in tissue culture. 



The critical size limit of an animal is subject to variations within any 

 species of animal. There are genetic strains in almost every species which 

 change this critical size, so that there may be varieties which are very small — 

 dwarfs — and other varieties which are very large — giants. These variations 

 are in number of cells, not size of cells. 



Growth of organs 



Now let us break down this over-all growth of the embryo into the 

 growth of individual parts. The over-all growth of the embryo is the summa- 

 tion of the growth of each organ. Organs grow at different rates. These organ 

 growth rates are subject to modification. If an organ is transplanted from one 

 embryo to another, its rate of growth may be altered. The growth rate is ap- 

 parently influenced by the nutrient medium. If an organ is transplanted into 

 an embryo where the concentration of nutrients is higher, the growth rate 

 may be higher. 



The growth of individual organs is due to an increase in the number of 

 cells. Since animal cells do not vary much in size, growth in an organ gen- 



