I06 CICATRIZATION OF WOUNDS 



characteristics. This fundamental discovery was not accepted 

 without a struggle by certain biologists, partioilarly in France. 

 A few scientists pretended that it was impossible, simply 

 because their own imperfect techniques had prevented them 

 from succeeding. These facts, however, are now classical and 

 no longer discussed by any one. 



I have spoken of 'technique'. It will be necessary to say a 

 few words on the subject so as to enable the reader to follow 

 the mechanism of the experiments on ageing which will be 

 described farther on and which established the link between 

 the experiments on cicatrization and those on tissue-cultures. 



The subject of tissue-culture techniques is vast enough to 

 fill a voluminous book written by Fischer, and I shall therefore 

 make a very brief summary. The principle is simple. It 

 consists in cutting out a fragment of living flesh and incor- 

 porating it to a medium capable of sustaining and nourishing 

 it. The products of excretion are eliminated by washing every 

 other day and the cultures, divided in two equal parts by 

 cutting with a sharp knife, are then transplanted separately 

 into a new medium. One of them serves as control. The 

 cultures are naturally maintained at the temperature which is 

 normal for the animal from which they proceed (about 38° C. 

 or 100° to 103° F. for chicken em^bryos). 



Realization is less simple. Indeed, as I have already stated, 

 the most important thing was first to find the nutritive medium 

 capable of indefinitely prolonging the life of the cells. Carrel's 

 idea of using embryonic juice was a stroke of genius and laid 

 the foundation of the whole method. The 'juice' is obtained 

 by mincing eight-day-old chicken embryos. To this end, 

 fecundated eggs are maintained in an incubator and, after 

 eight days, are brought into the operating room, which must 

 be as dust-free as possible. The operator, masked and in a 

 sterile blouse, breaks the shell after having washed and 

 sterilized it, and extracts the embryo which he cuts into small 

 pieces. These pieces are then crushed (in a 'Latapie' crusher, 

 for instance) and form a kind of pulp which is mixed with a 

 complex, rigorously balanced saline solution. This mixture 



