Genie Control of Development 403 



that is, in showing a fixed pattern (details in Wright, 1925). Many 

 genetic explanations for the action of the spotting factor have been 

 tried. It seems that Billingham and Medawar (1948, 1950) have found 

 a very unexpected solution. They observed in the skin a system of 

 colorless dendritic cells which have a definite arrangement and are in 

 contact with each other. Grafting experiments involving black and 

 white skin show that black cells can "infect" the white cells with 

 what is probably an enz\Tne system, behaving Hke a transferable virus. 

 The infected cells form pigment and are now able to infect other 

 white cells. Thus a genie action, production of melanin, is not locaHzed 

 in individual cells as in Drcsophila; it does not produce a diffusible 

 substance which acts like an inductor over a large area; but it produces 

 a cytoplasmic condition which spreads in a way comparable to an 

 infection. This is so far a unique t)^e of secondary' genie action of the 

 inductor or hormonic t>'pe, based upon a ver\^ different principle, 

 though it might be possible to find analogies with Holtfreter's X-sub- 

 stance (see III 5 A Z?). 



There can be no doubt a priori that inductor actions are under 

 genie control as to time and place of release and, probably, also as to 

 some kind of specificit}', meaning that a lens inductor is somehow 

 different from an inductor of mouth parts (see III 5 A b). But, un- 

 fortunately, a genetic analysis of inductor action is not available, if 

 possible at all, in view of the interchangeabiHt}' of inductors between 

 classes of vertebrates. Thus the genie control of inductive processes 

 can at present be attacked only from the side of the induced features 

 (e.g., competence of a tissue for induction). This side of the problem 

 has been analyzed in a former chapter. 



The second t>'pe of determinative short cut is by means of the 

 production of hormones. We know that genuine hormones of the 

 vertebrates determine some developmental processes; the classic 

 example is the role of the thyroid (and h\-pophysis) in producing 

 metamorphosis in Amphibia. In this case also, general genetic differ- 

 ences are known, though not analyzed by genetic methods, sho\\ing 

 different genetic reactivity' to the stimulus among different species. 

 This requires a system of genie action in which the production of the 

 proper hormones at the proper time enters as one variable with, an 

 over-aU effect as opposed to the localized effects of the basic genically 

 controlled reactions. A similar case might be made out for the sex 

 hormones in amphibian development, though there are diflBculties in 

 interpretation. The genetic analysis of frog races, different in regard 

 to their sex-determining mechanism (R. Hertwig, Witschi), has, it 



