472 PATTERNS AND PROBLEMS OF DEVELOPMENT 



It is sufi&ciently evident from this list that abiUty to bring about the 

 change in urodele tissues that constitutes induction is not hmited to any 

 particular kind of tissue from any single group of organisms but is a very 

 general property of tissues, either alive or dead or both. Holtfreter con- 

 cluded from his experiments that induction is effected by a chemical sub- 

 stance or substances. If a single specific substance is the inducing factor 

 in all cases, it must be a substance present in tissues of many kinds from 

 many groups, but the experiments give no clue as to what substance or 

 substances may be concerned. They do not show whether living and dead 

 inductors act in the same way or whether any of the foreign inductors in- 

 duce in the same way as the natural inductor. 



Quite aside from the demonstration of the widespread occurrence of in- 

 ducing power, these experiments, particularly Holtfreter's extensive data, 

 are of much interest in relation to certain other aspects of the problem of 

 induction. Since certain of the inductors bring about extensive inductions, 

 while others under similar conditions have only slight inducing action, it 

 appears probable that size and completeness of development of the in- 

 duced structure may indicate, in some degree, the intensity or effectiveness 

 of the inducing action. Frequency of induction also varies with different 

 inductors and may likewise indicate difference in effectiveness. In general, 

 inductors which become such only after killing are apparently weaker in 

 action than the natural inductors. Heating to ioo° C. usually weakens 

 inducing action of tissues that induce at lower temperatures, and heating 

 above 120° abolishes it in various cases. Invertebrate tissues are usually 

 less effective than those of vertebrates. Certain tissues — for example, 

 those of the internal glands, such as the liver — are more effective than 

 various others. Almost all the tissues used induce neural tissue, but only 

 certain of them induce mesoderm. The inductions differ greatly in char- 

 acter, ranging from mere epidermal thickenings or sometimes a single lens 

 or balancer to large masses, consisting of various supernumerary organs, 

 several neural tubes being formed in some cases. The neural induction 

 may resemble a part of the spinal cord or may become brainlike ; and gang- 

 lia, nose, eyes, otic vesicles, and balancers may develop in relation to it. 

 Supernumerary legs may also develop, and induction of muscle and noto- 

 chord from ectoderm or from mesoderm sometimes occurs. 



The apparent absence of any definite relation between character and 

 origin of inductor and character of parts induced suggests that the foreign- 

 tissue inductors are primarily activators rather than definitely specific for 

 particular organs or complexes and that other factors — physiological con- 



