FORMATION OF STAPEDIAL PLATE 105 



tion. Werber has shown that numerous isolated lenses may be 

 developed m chemically treated teleost embryos in the absence 

 of optic vesicles. Even here we may have a response to stimuli 

 exerted by disrupted diencephalic tissue. The evidence in favor 

 of self-differentiation of the lens seems to be entirely outweighed 

 by the production of lenses in foreign ectoderm in the experiments 

 of Lewis. Spemann and Lewis have also shown that the forma- 

 tion of the cornea is a response to stimuli from the optic cup, 

 and will take place even though the lens be removed; that the 

 cornea, once formed, degenerates with the removal of the optic 

 cup. The work of Lewis on the formation of the amphibian otic 

 capsule has already been mentioned. In a recent communica- 

 tion, I believe I have demonstrated that myocardial concres- 

 cence is a tactic response to the presence of endocardial tissue. 

 Finally the stapedial plate is of interest in this same connection. 

 If these several observations be confirmed, investigations of this 

 sort may prove to be of extreme interest. It is probable that 

 the interaction of parts is of greater importance than we are 

 generally aware. In many cases we have reason to believe that 

 in the earliest stages in the formation of many organs, a stage of 

 equipotentiality of parts is not widely departed from, but that 

 as development proceeds, totipotence is lost and powers of self- 

 differentiation are more strongly asserted. In general one should 

 expect from this that 'dependent development' would be a 

 phenomenon confined to the earlier stages of ontogeny; there 

 seem, however, to be certain exceptions to this. Of these ex- 

 ceptions, two sorts seem to be well established. First, it is a 

 striking fact that many of the cases of 'dependent development' 

 above discussed have to do with relatively late organ-formation. 

 In fact it seems reasonable to suppose that the action of a dif- 

 ferentiated part upon an undifferentiated one would be greater, 

 the farther the activity of the former had advanced. The cases 

 cited have to do with organ-formation in which the process is 

 one of synthesis or 'composition' of rather diverse sorts of tissue. 

 A second exception is that of non-differential cleavage or non- 

 differential development in which an apparently indifferent tissue 

 sometimes maintains itself until relatively late in ontogeny, re- 



