114 CICATRIZATION OF WOUNDS 



not nourish normal cells. It is therefore possible to conclude, 

 as does Dr. Carrel, that cancerous cells acquire their malig- 

 nancy in vivo because of their faculty of manufacturing 

 nutritive substances from the surrounding tissues and fluids. 

 This enables them to proliferate in an unlimited fashion. 

 The mutability of certain types of cells, which we spoke of 

 above, takes on all its importance at this point. Malignant 

 cultures are varieties of normal types from which they differ 

 only slightly through certain properties. These differences 

 are not qualitative but quantitative and irreversible. After 

 several years of culture in vitro they have not regressed to the 

 original type. They are fixed varieties. 



The hypothesis of the microbic nature of cancer has been 

 definitely abandoned as a result of these experiments and of 

 many others which it would take too long to describe in 

 detail. Unfortunately, the knowledge thus obtained has not 

 yet resulted in any practical progress towards the eradication 

 of this scourge. But these researches have only started and 

 every hope is permissible. 



The almost unlimited applications of this method are easy 

 to conceive. Thanks to it we can study the problems of 

 immunity. A tissue behaves like an organism and reacts 

 against the poisoning due to the toxic substances carried by 

 microbes by manufacturing the specific antidote substances 

 called antibodies. From a practical point of view. Carrel and 

 Rivers succeeded in cultivating smallpox vaccine by inocu- 

 lating it to cultures of cornea, skin, and embryonic tissue. 

 The virus multiplies rapidly, and it is probable that a chicken 

 embryo reduced to a fine pulp can produce as much vaccine 

 as a calf. 



The reader had to have precise notions on tissue-culture in 

 general so as to be able to follow the development of the 

 technique employed in checking the method of quantitative 

 study of cicatrization expounded in the preceding chapters. 

 That is why I have kept this problem for the end. 



Dr. Carrel was induced to study these problems by former 

 experiments on the ageing of animals. These experiments 



