582 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1910. 



It was attempted also to study the action of the changes in the 

 composition of the medium on the rate of growth of normal tissues. 

 Slight modification of the tension, or of the alkilinity, or of the con- 

 centration of the inorganic salts modified the rate of growth. It 

 may be possible to discover by this method the physicochemical fac- 

 tors which regulate the growth of each kind of tissue. 



It would be important to know why and how a wound heals. The 

 laws of cicatrization are actually unknown. Surgeons content them- 

 selves by preventing the infection of the wounds and leave to nature 

 the care of healing it. However, if we knew the physico-chemical 

 mechanisms, which, coordinated by the power of redintegration 

 acting as a directing idea, bring about the healing of a wound, we 

 might act on the process of the cicatrization itself and activate it. 

 The knowledge of these laws would lead to a new and more effective 

 form of surgery. But this study is very difficult on living animals, 

 while by cultivating tissues in vitro, in a given medium, it becomes 

 possible to observe exactly the modification in the rate of growth 

 under the influence of certain substances. In this laboratory. Dr. 

 Ruth has observed that small wounds made in the center of a frag- 

 ment of skin heal normally in vitro. All the stages of the cicatriza- 

 tion can be followed. The edges of the wounds are brought together, 

 and at the same time, the epithelium proliferates, and a complete 

 epidermization of the surface of the small wound occurs. This 

 method permits us to observe very easily the different stages of the 

 cicatrization and the modifications of it produced by the changes in 

 the composition of the medium. 



The solution of various problems may be helped by the use of this 

 method of cultivation of tissues in vitro, because it renders possible 

 the observation of cells growing under given conditions. It is a new 

 instrument which can be used in the study of the mechanisms of 

 cellular growth and of its unknown laws. 



