SOCIAL LIFE OF ANIMALS 495 



Ducks. — The wild duck, Anas boschos, the ancestor of our domes- 

 tic species, is found both in China and in Europe. 



Geese are the oldest birds tamed by man. The ancestor of our 

 races is Anas cinereus. The early Egyptians tamed the Nile goose, 

 Chenalopex egyptiaca. 



Peacocks. — Domesticated peacocks have descended from the 

 wild Indian species, Pavo cristatus. 



Turkeys have descended from two wild North American species, 

 Meleagris gallopavo and a Mexican variety. We now have four wild 

 sub-varieties. (Robertson.) 



Tissue Survival Outside the Body 



The pioneer in tissue culture, Dr. R. G. Harrison of Yale, showed 

 many years ago that fragments of frog embryos isolated from the 

 organism would develop for a number of days. His classic experi- 

 ment on the development o{ neuroblasts in a hanging drop paved the 

 way for much important experimentation. Montrose Burrows was 

 another important figure in the development of the technique of 

 tissue culture. 



Dr. Alexis Carrel, of the Rockfeller Institute, started the culti- 

 vation of minute fragments from the heart of a chick embryo on 

 January 17, 1912.^ Since that time he and his associates have 

 carefully subdivided the culture, washed it in Ringer's solution and 

 cultivated it again in a medium composed of diluted plasma. 

 Carrel states that if the tissues had been able to survive for 16 years 

 without trimming or hindrance, the mass would now be greater 

 than that of the solar system. He emphasizes the fact, however, 

 that cells massed together as a closed system must necessarily under- 

 go the process of aging. 



The immortality of protozoa (see page '^i^^ is due to their ability 

 to eliminate products of metabolism directly into the outside world. 



Carrel mentions the property of cell proliferation found in iris 

 epithelium and thyroid epithelium, which have been cultured at the 

 Rockefeller Institute for several years. 



5 Carrel, A. 1924. Tissue culture and cell physiology. Physiol. Rev., vol. 4, no. 



I, pp. 1-20. 



Fischer, A. 1925. Tissue Culture. Wm. Heinemann, London. 



Sundwall, J. 1912. Tissue Proliferation in Plasma Medium. Bull. no. 81, 

 Hygienic Laboratory, U. S. P. H. S. 



Willmer, E. N. 1928. Tissue culture from the standpoint of general physiology, 

 Biol. Rev., vol. 3, no. 4, pp. 271-302. 



