THE CULTIVATION OF TISSUE IN VITRO 457 



lost in differentiation, as nerve and muscle cells. Such 

 cells or tissues containing such cells would, therefore, be 

 manifestly ill adapted to such experiments as are under 

 consideration, except during the period of embryonal 

 development, when they multiply actively like other tissue 

 elements of less specialization. 



From these and similar conditions that may suggest 

 themselves it becomes, evident that to test the viability of 

 fragments of tissue independently of the body as a whole, 

 choice should be made of : 1, embryonal tissues whose chief 

 activity is vegetative and reproductive; 2, adult tissues of 

 low specialization whose cells easily multiply and are 

 known to endure adverse conditions easily; 3, tissues of 

 tumors, whose tissues have an abnormal tendency to 

 multiply. 



It is difficult to find when, how, and by whom the 

 earliest experiments of this kind were made, but probably 

 few attempts were made before 1892 when Leo Loeb, 

 interested in the study of cancer, was led to try whether 

 it was possible to make skin grow under artificial con- 

 ditions. He used for the experiment bits of the skin of 

 embryo guinea-pigs which he embedded in agar-agar and 

 then inserted in the tissues of adult guinea-pigs. The 

 agar-agar kept them separate from the tissues of the host 

 animal and defended them from the attacks of phagocytes, 

 etc., while they were supplied with nourishment by the 

 lymph of the host. Under such conditions the tissues 

 survived for a time and appeared to grow, as mitotic 

 changes and wandering of the cells were observed. This, 

 however, was not an experiment in vitro, and more nearly 

 resembled grafting. 



The chief researches on the subject took place between 

 1907 and 1912. They were begun by R. G. Harrison and 

 later continued by Burrows, Carrel and Burrows, Lambert 

 and Hanes, Ruth, Jolly, Murphy and Rous, Carrel and 

 Ingebrigsten, Lewis, Foot, Eveling, Hyami and Tanaka, 

 and others. 



Harrison's researches paved the way for the subsequent 



