Introduction 5 



then no modification of a given cell-line 's environ- 

 ment could hope to restore the lost functions. It 

 is clear that a decision between these two alterna- 

 tive hypotheses is essential to any valid notion of 

 the origin of form and function. And it is also 

 clear that one of the most promising techniques 

 for arriving at such a decision lies in the segrega- 

 tion of cells, tissues, and organs from the associ- 

 ated members of the body and their maintenance 

 and study as isolated units, under as nearly opti- 

 mal and as fully controlled conditions as possible. 

 The attempt to reduce an organism to its constitu- 

 ent cells and to study these cells as elementary 

 organisms is thus a project of fundamental impor- 

 tance in the solution of basic biological questions. 

 Schleiden (1838, 27) and Schwann (1839, 29), 

 who formulated the concept of cellular totipotency, 

 and Virchow (1858, 433), who popularized that 

 concept, made no attempt to put it to experimental 

 test. A half century elapsed between its formu- 

 lation and the first well-organized experimental 

 study of the question. In the 1870 's, several 

 workers attacked the problem, no one with more 

 lucidity and patience than Vochting. Vochting 

 chose two methods of approach. First, he dis- 

 sected plants into smaller and smaller fragments, 

 studying the phenomena of polarity in these f ra, 

 ments. He found "polarity" to be a chara 



