II. THE PERIOD OF ORGAN DEVELOPMENT 135 



the ectoderm is necessary for the induction of a lens to take 

 place. If the contact is locally interrupted by the insertion 

 of a strip of cellophane, no lens placode is formed at that 

 place. This has led P. Weiss to the conclusion that lens in- 

 duction is not due simply to the diffusion of an "evocator", but 

 rather to a contact phenomenon, e.g. a stereochemical reaction 

 in the interface between inductor and reaction-system. However, 

 recent data from Woerdeman's laboratory cannot be brought 

 into full agreement with this view. Evidence was obtained there 

 by means of serological methods that specific lens proteins 

 are already present in the lens-primordium at the lens-groove 

 and lens-vesicle stages, i.e. before the beginning of actual fibre 

 differentiation (Ten Gate and Van Doorenmalen, 1950). Further- 

 more, Woerdeman (1950) demonstrated that such lens proteins 

 are also formed in mixtures of extracts of eye-vesicles and of 

 head-ectoderm from early embryos in which no lens formation 

 has yet begun. Here the reaction concerned evidently takes 

 place quite independently of the normal structure of the cells. 



A further group of experiments was made on the origin of 

 the cornea. Both the eye-cup and the lens may induce the 

 formation of a cornea. In normal development, therefore, this 

 tissue is apparently formed under the combined influence of 

 both factors (Lewis, 1905; Fischel, 1915-19). The whole skin 

 ectoderm of the larva has the capacity to differentiate into a 

 cornea under the influence of these inductions. The presence 

 of the eye is necessary, not only for the development of a 

 cornea, but for its maintenance as well. After the extirpation 

 of eye-cup and lens, de-differentiation of the cornea takes place, 

 and it alters into normal skin. Here, again, the induction is not 

 species-specific. 



It is clear, therefore, that the normal development of the 

 eye is based on an interplay of inductions, whereby each part 

 influences the development of all others, and also reacts to the 

 influences from all others, all parts having acquired the ne- 

 cessary reactivity in their previous chemodifferentiation. 



Similar situations obtain in other organs. The olfactory organ 

 of the amphibians originates from a pair of thickenings of the 

 ectoderm in the anterior region of the head. Here, in front of 



