ORIGIN OF VASCULAR TISSUES 125 



— a phenomenon long since recognized and known, as indetermi- 

 nate development. The great importance and great complexity 

 of the intercelkdar environment is little appreciated. We are 

 likely to consider this type of environmental differences as of 

 little importance for the reason that we know little about them. 



Again I wish to state as I have twice done previously that 

 "the development of the vascular system furnishes an unproduc- 

 tive field for the solution of the problems of preformation and 

 epigenesis." A diffuse tissue like the mesenchyme is the very 

 worst sort of tissue in which to prove predestination by authority. 



Despite the very important work which has recently been done 

 on the vascular system, the interest which attaches to its main 

 problems is by no means merely historic. The vascular system 

 still may have some surprises in store for us. The fact that the 

 problem can be attacked experimentally need by no means be- 

 little its importance. As yet we are still in the stage of testing 

 our methods, and will do well to accept with caution the results 

 of experimental methods, remembering that experimental con- 

 ditions may obscure the normal process. The very best we can 

 do experimentally is to subject the embryo to as many methods 

 of experiment as possible, selecting results from those methods 

 which alter to the least extent the normal process. By com- 

 bining the pictures obtained from these various methods into a 

 composite picture we may by chance obtain a resultant which will 

 be of service to us in the study of the normal process. By de- 

 termining as many as possible of the things which a tissue is able 

 to do, we obtain information as to what it does do. We may, 

 perchance, sometimes find that a given tissue can do some of 

 those things which it had previously been regarded as unable to 

 do; this alone would justify experimental effort. 



I wish to make clear that in no case do I regard an experimental 

 condition as portraying a truly normal process, and have there- 

 fore not volunteered an estimate of the significance to be at- 

 tached to the observed conditions in each expei'imental case. 

 It is too much to dare hope that my results will meet with ready 

 acceptance; all I can say is that during the time in which I have 

 worked, I have spared neither labor nor pains to make these re- 

 sults accurate. 



