212 HUMAN BIOLOGY 



multiplication must always be sought in the nature of the 

 surrounding fluid. The growth energy of a cell at a given 

 instant is a function of its inherent growth energy at the 

 preceding instant and of the concentration of growth- 

 promoting and growth-inhibiting substances in its medium. 



3, Diversity of Growth-Promoting Factors According to Cell 

 Types. Pure strains of epithehal cells or fibroblasts, when 

 placed in a medium composed of embryonic proteins, or of 

 proteoses, peptones, and peptides, begin at once to multiply. 

 Adult as well as embryonic cells respond in identical ways 

 to the presence of these substances. The rate of growth 

 depends both on their nature and their concentration. But 

 fibroblasts, epithelial cells, and macrophages do not behave 

 in the same manner toward a given substance. In embryonic 

 proteins, the rate of proliferation of epithelium is always 

 slower than that of connective tissue. Thyroid cells, and 

 iris or Malpighian epithelium in pure cultures grow much 

 less actively than fibroblasts. Moreover, these cell types do 

 not utilize serum proteins. When cultivated in such a 

 medium, they die within a few weeks while, on the other 

 hand, blood and tissue macrophages proliferate rapidly in 

 serum. The latter also multiply when fed on muscle frag- 

 ments or protein precipitates. They remain in the localities 

 where these particles are present and increase in size, as 

 w^ell as in number. But in digests from proteins and in con- 

 centrated solutions of embryonic proteins, they do not 

 multiply, and often die. If such an investigation were 

 extended to other cell types, differences doubtless would 

 be discovered also in the nature and concentration of the 

 substances w^hich promote their growth. The innate prop- 

 erties of the various cell types account for the specific 

 response of the tissues within the organism toward a given 

 nutrient substance. Their activity is automatically and 

 differentially determined by the quantity and the quality 

 of the food supply. 



Some nutrient substances may be manufactured by the 

 tissues themselves. For instance, tissue cells cultivated in a 

 flask set free in their fluid medium growth-activating 

 substances. When leucocytes are multiplying actively in a 

 plasma coagulum, the medium acquires the power of pro- 



