TISSUE CULTURE 67 



Harrison succeeded in doing with animal material what 

 Haberlandt predicted should be possible and thus paved the way 

 for all of the remarkable work in tissue culture which has since 

 been done by Carrel and Lewis in America, Strangeways in 

 England, Fisher in Denmark, Levi in Italy, and others. 



Harrison was interested in the development of the nervous 

 system. He realized that there was need for a method in which 

 the behavior of certain cells could be observed when removed 

 from the bewildering conditions existing within the body. The 

 ordinary methods of histology were inadequate to answer the 

 question of the origin of the nerve fiber. Here was a problem 

 that demanded for its solution some new form of technique, a 

 method that would permit the growth and study of isolated 

 nerve cells. Consequently, in 1907, Harrison placed small 

 portions of the various tissues of the frog embryo in drops of 

 lymph suspended over the depression in a hollow-ground glass 

 slide. The lymph coagulated and formed a substratum, and the 

 cells grew. During the days that followed, Harrison observed 

 the formation of protoplasmic threads from the cells and was 

 thereby able to show that embryonic nerve cells form long proc- 

 esses which can be identified with nerve fibers in the embryo. 

 The fiber was seen to develop from the spinning of a thread of 

 protoplasm by means of the amoeboid activity of the free end 

 of a cell process. 



Others before Harrison, in addition to Haberlandt, had the idea 

 of isolating tissues and studying their behavior. Among these 

 were Wilhelm Roux and Leo Loeb. The latter planted pieces 

 of epithelium from the guinea pig in small blocks of clotted blood 

 or agar, which were then placed for incubation in the body of 

 another animal and subsequently studied by means of micro- 

 scopic sections. But this procedure is very different from that 

 of tissue culture as at present practiced and can hardly be said to 

 have suggested it, although the underlying purpose was the same. 



The experiment which Haberlandt attempted and Harrison 

 brought to fulfillment Carrel perfected. Simply put, the tech- 

 nique consists in isolating a small bit of tissue, either from an 

 animal embryo or from the actively growing tissues of an adult 

 body, and placing this bit in a suitable medium. The prepara- 

 tion is then hermetically sealed, placed in an incubator at body 

 temperature, and allowed to grow. The tissues most frequently 



