72 PROTOPLASM 



do they grow, i.e., increase in number; cell division rarely occurs. 

 Cells live in pure salt solutions only as long as the residual energy 

 (stored reserves) with which they were endowed when they were 

 placed in these solutions lasts. When the food reserves have 

 been used, no further growth can possibly take place in the 

 absence of utihzable proteins. Distinction must, therefore, be 

 made, in the case of animal cells, between a nutritive medium 

 (containing protein or protein derivatives) and a "protective" 

 medium such as Ringer's solution or sea water. In drawing 

 such a distinction, one must remember that it is always exceed- 

 ingly difficult to decide in the last analysis between what is 

 nutritive and what protective. It cannot be arbitrarily stated 

 that all inorganic elements are protective and all organic ones 

 nutritive or that known nutritive compounds are not also pro- 

 tective. Yet it is true that in certain (inorganic) solutions 

 animal cells live for a time but do not grow. We may regard 

 these as protective. It becomes necessary, therefore, to find a 

 culture medium that will promote growth. Again Carrel had a 

 happy thought. He added to the cultures a drop of embryonic 

 juice, the fluid taken from crushed embryos and therefore the 

 same kind of fluid in which the tissue is bathed when it is growing 

 in its normal place in the developing organism. The result was 

 most successful. We shall let Carrel tell about it in his own 

 words : 



A truly wonderful effect was immediately observed. Fibroblasts 

 began to multiply about the tiny pulsating heart muscle, which was 

 soon surrounded by a large amount of tissue in which it disappeared. 

 The tissue went on growing and could be divided into two parts, which 

 also grew rapidly. Every forty-eight hours, the cultures were washed 

 in Ringer solution by Ebeling or myself, divided into two parts, and 

 cultivated again in embryonic juice. Today, hundreds of experiments 

 are made every month with the pure strain of fibroblasts descended 

 from the tiny fragment of pulsating tissue that I possessed in 1912. 



A strain of cells (fibroblasts) derived from this bit of embryonic 

 heart muscle started by Carrel on Jan. 17, 1912, is growing 

 today, twenty-three years later (Fig. 57). The rate of growth is 

 tremendous. The number of generations is now over 4,000. 

 Each cell divides, on an average, twice in forty-eight hours, thus 

 doubling the size of the colony. 



