Methods and Techniques 



35 



quite other than those it would have formed 

 in the normal embryo, hence Bautzmann's 

 term bedeutungsfremde Selbstdifferenzierung. 

 The occurrence of these and other examples 

 of bedeutungsfremde Selbstdifferenzierung 

 signifies that the cells have sufficient plas- 

 ticity to differentiate in other than their 

 normal direction as a result of change in 

 the conditions with reference to which they 

 are differentiating, but is not adequate to 

 define the changes which have produced an 

 alteration in the direction of differentiation. 



It has been postulated that one way, theo- 

 retically, to come closer to a definition of 

 conditions essential to differentiation might 

 be the testing of differentiation capacity in 

 the widest possible variety of media. But if 

 the reactions and directions of differentiation 

 should be studied under as many experimen- 

 tal conditions as possible, how is the infor- 

 mation to be referred to the cells in action in 

 the normal embryo? Observations woidd be 

 available as to media satisfactory to elicit all 

 ranges of differentiation; but would not the 

 ascertained data remain knowledge of re- 

 actions in an external medium rather than 

 of processes internal to the embryo itself? 

 The comparison of different results in various 

 solutions fails to permit reference of the ex- 

 perimental results to the processes which 

 might have been carried out by the cells 

 in situ. What is the way to prove in the 

 isolation experiment whether the cells which 

 have differentiated particular structures are 

 the same which would have done so in the 

 embryo or whether they have used the same 

 method to reach their end? 



If, however, the processes studied experi- 

 mentally cannot be referred without caution 

 back to the normal embryo, it is clear that 

 the processes examined experimentally re- 

 quire rigid definition as to the conditions 

 under which they occur. No medium can 

 be neutral or indifferent with respect to 

 early differentiation; if it were actually 

 either of these, isolates could continue no de- 

 velopment at all; a medium can be neutral 

 or indifferent only to a cell that is dead. 

 The failure to come to grips with the prob- 

 lem of relationship between embryo and 

 medium grows partly out of the tendency to 

 emphasize the degree to which cells "self- 

 differentiate." The concept of self-differenti- 

 ation implies a contradiction in terms; no 

 cell can "self-differentiate, bedeutungsfremd 

 or bedeutungsgemdss, insofar as no cell can 

 be separated from its environment. 



Roux expressed the problem more cogently 

 when he opposed dependent and independent 



differentiation, a classification which de- 

 mands enumeration of the factors with ref- 

 erence to which differentiation might be de- 

 fined as either of these. One of the strong 

 needs of the moment is more specific defini- 

 tion of the conditions with reference to 

 which both normal and abnormal differen- 

 tiation occur; only this will permit evalua- 

 tion of the meaning of changes in the differ- 

 entiating systems proper, since only in this 

 way can embryologists be certain when they 

 are dealing with these systems themselves 

 and not something extraneous to them. 



While attempts to invent the medium 

 ideal to support specific types of differentia- 

 tion may suffer in that the results of the 

 studies lack referability to the normal em- 

 bryo, the attempts must still be continued, 

 though as a means to an end. Only when 

 knowledge is available concerning the re- 

 actions of the cells to the media in which 

 they develop can other experiments be inter- 

 preted which are conducted in other ways to 

 answer the more searching questions which 

 remain at the backs of our minds; but the 

 fact must not be lost sight of that these more 

 fundamental questions remain to be asked 

 by other experimental methods. 



The remaining one of these methods to be 

 discussed is the transplantation method, 

 which allows the framing of different ques- 

 tions than those methods already discussed, 

 or which perhaps rather allows comparable 

 questions to be asked in a slightly different 

 way. 



Strictly speaking, the implantation of cells 

 to such cellular environments as the chorio- 

 allantois, the eye chamber and the other sites 

 used for studies in vivo might be considered 

 either as transplantation or as explantation 

 experiments; and perhaps some of the best 

 uses to which the transplantation method 

 has been put have been those in which it 

 has been employed as an isolation method, 

 as in the case of Harrison's ('03) early ex- 

 periments on the lateral line. 



When cells are isolated in culture in some 

 synthetic medium, the direction of their dif- 

 ferentiation presumably may be influenced 

 by the reaction of cells to factors in the 

 medium; if they are transplanted to a new 

 cellular environment, it will be conditioned 

 in relationship to factors emanating from 

 neighboring cells, and therefore by mutual 

 interactions between cells of graft and cells 

 of host. 



When cells are transplanted to a cellular 

 environment they are transferred, as in 

 many explantation experiments, to a milieu 



