1 6 PROTOPLASMIC ACTION AND NERVOUS ACTION 



protozoa remain as single cells throughout life. The 

 higher animals and plants are single cells only at the 

 beginning of their development — in the germ-cell stage; 

 in later developmental stages and as adults they consist 

 of large, closely associated aggregates or colonies of 

 cells, which, together with the intercellular fluid media 

 serving for transport (blood) and various other products 

 of cellular activity (skeletal and other structures), form 

 a complex and highly integrated system, or organic 

 individual. Each cell in this organism is to be regarded 

 as hving and capable of independent existence under 

 appropriate conditions. 



The statement that single isolated cells are capable of 

 independent life has been shown experimentally to be 

 true, not merely of organisms which throughout their 

 life are unicellular, but also of many of the cells of higher 

 organisms when isolated under favorable conditions — 

 leucocytes, ciliated cells, muscle cells, tissue-cells. 

 Epithelial cells will grow in suitable culture-media;^ 

 embryonic nerve cells, isolated in sterile plasma, send 

 out axones in a characteristic manner; i.e., retain the 

 normal power of growth, differentiation, and develop- 

 ment;^ and many functional adult cells continue to 

 live and grow when isolated under favorable conditions 

 of food and oxygen supply.^ 



On the other hand, experiment shows that for normal 

 and long-continued vital activity the cell must be 



^ Cf. L. Loeb, Arch. Entwickl. Organ., XIII (1:902), 487, and earlier 

 papers there cited. 



'Harrison, Proc. Soc. Exp. Biol, and Med., Ill (1907), 140; Journal 

 of Experimental Zoology, IX (1910), 797; XVII (1914), 521. 



3 Cf. Carrel and Burrows, Journal of Experimental Medicine, XIII 

 (191 1); W. H. and M. R. Lewis, Anatomical Record, VI (191 2). 



