210 HUMAN BIOLOGY 



no experiment can be made on a non-physicochemical agent 

 distinct from the body, it is entirely feasible to measure an 

 impulse toward organization present in a small group of 

 cells. It has recently been discovered that cells removed 

 from their normal surroundings and caused to hve as 

 independent units begin at once to manifest their innate 

 properties. The analysis of these properties, which remain 

 hidden in normal hfe, may explain the mechanism of some 

 of the formative and regenerative processes. Such an investi- 

 gation can be made on embryonic as well as on adult organ- 

 isms. It is well known that the prospective value of any 

 group of embryonic cells is far greater than its real value. 

 When a blastula is cut into two parts, each develops an 

 embryo. This experiment indicates that the fate of a cell 

 is a function of its position. The egg is an equipotential 

 harmonious system, as Driesch has named it. Each element 

 appears to be able to play diflerent parts equally well in 

 the formation of the totahty. What factors are responsible 

 for the actuahzation or non-actuahzation of its potential- 

 ities? Probably certain chemical substances set free by the 

 cells themselves. The epidermis of amphibia produces the 

 lens of the eye under the influence of a formative stimulus 

 from the primary optic vesicles. The analysis of the nature 

 of such a stimulus is impossible when the tissues are parts 

 of a living organism. But it would become feasible if the 

 physicochemical conditions that may determine the trans- 

 parency of epidermis were ascertained in vitro, and if the 

 substances set free by optical vesicles were studied under 

 the same conditions. 



Should the principles determining organization reside 

 within the elements composing the body, they would become 

 apparent if tissue cells of various types were isolated and 

 maintained in vitro in a condition of active and free life. 

 With this object in view, we have developed elaborate 

 physiological techniques during the last few years by which 

 tissues and blood cells can be separated from the body and 

 caused to show their natural tendencies toward organization 

 and the elemental properties underlying formative, regenera- 

 tive and, adaptive stimuli. The application of the method of 

 tissue culture in its modern form to embryonic and adult 



