TISSUE-CULTURE IN VITRO III 



multiplication requires the presence of a certain number of 

 cells in the same medium. Everything occurs, in brief, as if 

 certain substances secreted by the cells were required to set 

 free the mechanism of proliferation of the other cells. In 

 other words, they have to be fecundated. What are these sub- 

 stances; what is the mechanism? Can it in some way be com- 

 pared to a particular form of sexuality, or is this phenomenon 

 connected with the oxidation-reduction potential of the 

 medium? All these problems are still very mysterious. 



We have insisted on the necessity of employing pure cul- 

 tures, composed of a single type of cell. This is a capital point, 

 but also the most delicate of the method. Its importance was 

 not fuUy realized by many workers who at the start were filled 

 with enthusiasm by the possibilities which they foresaw. Some 

 of them, when they finally understood, were soon discouraged 

 by the meticulous care, the patience, the time, and the techni- 

 cal equipment required to conduct successfully constructive 

 experiments. The tissue-culture laboratory at the Rockefeller 

 Institute comprises about fifteen people: experimenters, 

 assistants, women technicians, and orderlies. 



Even though certain species of cells can now be isolated by 

 well-established techniques, there are other varieties which 

 have only been isolated once or twice, thanks to a sequence of 

 circumstances not always reproducible. One or more factors 

 remain undetermined. The general method of selection con- 

 sists in finding culture media of such composition that they 

 will favour the proliferation of the cells which are to be isolated 

 whilst discouraging the others. Small variations in the con- 

 centration of the embryonic juice, addition of blood-serum, 

 and modification in the rhythm of the transplantations are 

 amongst the principal means employed. But there are others, 

 depending on the nature of the cells. Epithehal cells, for 

 instance, live only on condition that they are placed at the 

 surface of the coagulated plasma drop. Naturally it is im- 

 portant to obtain the fragments of tissue from the embryo in 

 as pure a state as possible. This is not always easy. The 

 dissection of a thyroid gland on an eight- or ten-day embryo 



