CONDITIONS OF CELLULAR IMMORTALITY 59 



including processes of growth and differentiation, the 

 power of survival of tissues and organs and their trans- 

 plantability to strange regions, even to other individuals, 

 has long formed the basis of practical procedures. " 



The first successful cultures of somatic cells and tis- 

 sues outside the body were those of Leo Loeb, described 

 in 1897. His first method consisted in cultivating the 

 tissues in appropriate media in test tubes. Later he used 

 also another method, which involved the transplantation 

 of the solid medium and the tissue into the body of an- 

 other animal. What has been regarded as a defect of 

 both these methods is that they do not permit the contin- 

 ued observation of the cells of the growing cultured tissue. 

 To Harrison is due the development of a method which 

 does permit such study. In 1907 he announced the dis- 

 covery that if pieces of the developing nervous system of 

 a frog embryo were removed from the body with fine 

 needles, under strictly aseptic precautions, placed on a 

 sterile cover slip in a drop of frog lymph, and the cover 

 slip then inverted over a hollow glass slide, that the tis- 

 sues would remain alive for many days, grow and exhibit 

 remarkable transformations. By tin's technique it was 

 possible to study the changes with a high power micro- 

 scope and photograph them. 



Figure 13 is a general view of one of these tissue cul- 

 tures two days old. It shows a piece of nervous tissue 

 from the frog embryo, with cells growing out from it 

 into the lymph. The lighter portions are the new cells. 

 In his remarkable monograph Harrison shows nerve 

 cells developing fibers at first thickened, but presently 

 becoming of normal character and size. At the ends are 

 pseudopodial processes, by which the growing fiber at- 

 taches itself to the cover slip or other solid bodies. Fig- 



