474 PATTERNS AND PROBLEMS OF DEVELOPMENT 



tissues may induce mesoderm and secondary appendages, but in general, 

 mouse kidney induces trunk parts less frequently than Triton liver. Ac- 

 cording to the author, a "host effect" masks more or less completely the 

 specific differences in inducing action of the two tissues. This host effect 

 consists in decrease in frequency of head parts from anterior levels poste- 

 riorly and in trunk parts in the opposite direction. Moreover, with mouse 

 kidney the character of induction differs after different periods of boiling: 

 with boiling for a few seconds, induction, especially of mesodermal parts, 

 is increased; after 15 minutes of boiling no mesodermal parts are induced, 

 and frequency of brain inductions is decreased; after i hour of boiling 

 there is further decrease in brain inductions, but sense organs and bal- 

 ancers are still induced. These results are regarded as due to different sub- 

 stances or as specific effects of different periods of boiling on some sub- 

 stance or complex. However, the evidence for specific action does not ap- 

 pear any more conclusive in these than in other experiments. The differ- 

 ences may be due to differences in intensity or rate of inducing action and 

 stage of development of host. The "host effect" is obviously an expression 

 of the longitudinal gradient. If the inducing action of kidney and liver 

 were actually specific, how could the host effect alter it to a mere differ- 

 ence in frequency of particular parts? The results after different periods of 

 boiling may also indicate merely different intensities or rates of inducing 

 action, effective at different stages of host development with different re- 

 active capacities of host tissue to the same inducing action. 



The conclusion that organ fields of the host determine what parts re- 

 sult from action of foreign inductors (Weiss, 1935) seems not entirely in 

 accord with the data. Very probably the fields are concerned in certain 

 cases — for example, when a lens or a leg is induced — but whether or to 

 what extent the various fields are present and sufficiently developed to de- 

 termine the result of induction is uncertain. While certain parts, such as 

 brain, appear more frequently in certain regions of the host, they may ap- 

 pear elsewhere. In many cases the region activated by the inductor ap- 

 parently develops organ fields of its own, as might be expected if fields re- 

 sult from a gradient pattern. 



In the light of the evidence considered thus far the question is pertinent 

 whether or to what extent these inductors, natural or foreign, living or 

 dead tissues, are actually organizers? It was pointed out above that the 

 longitudinal gradients of ectoderm and inductor coincide in direction after 

 invagination and more or less closely in extent. Does the chorda-meso- 

 derm do anything more in normal development than "reinforce" and per- 



