190 PHYSIOLOGICAL GENETICS 



The specific struct me of the testis, for example, makes an induc- 

 tion by simple contact rather improbable. After all, the differ- 

 ences between induction by diffusion and by hormones are found 

 to disappear the more we learn about induction. Two sets of 

 facts demonstrate this. Greenwood and Blyth (1935) showed 

 that the sex hormone in quantities below a certain threshold is 

 not distributed by the blood stream but diffuses only from cell 

 to cell; the same substance is then once inductor by diffusion 

 and again a real hormone. Another example relates to the other 

 end of the series: Goldschmidt (1921a) found that in intersexual 

 males of Lymantria, frequently duplications and triplication 

 of parts of the male genital armature (the valvae) appeared. 

 He concluded that the cause must be the increased breadth of 

 the region of formation of these organs caused by the larger size 

 of the intersexual abdomen. Recent work by Holtfreter (1936) 

 shows that this explanation was right and why. This author 

 showed that a stretching of the amphibian organizer resulted 

 in multiple inductions. We concluded from the facts mentioned 

 above that an induction of the hormonal type must be present 

 in these parts of the genital armature. Here we have another 

 proof, which in addition links the facts with the typical cases of 

 embryonic induction. 



3. Diffusion of Growth- or Similar Substances. — One of the 

 most frequent actions of the mutant gene is the production of 

 insufficient quantities (as compared with the normal gene) of 

 substances needed for normal differentiation. Lacking detailed 

 information, we might generally call them growth substances. 

 We have met them before in cases where possibly processes within 

 the individual cells were involved, e.g., the bristle-forming mate- 

 rials in Plunkett's study (1926). A case where we can hardly 

 escape an interpretation in terms of production of such sub- 

 stances, or their reciprocals, lytic substances, destroying previous 

 growth, was the scalloping effect of the vestigial series, and the 

 interpretation was already given in this sense. Here there can 

 be no doubt that the mutant development is either short in some 

 substance necessary for normal growth and differentiation; 

 furthermore, that this deficiency may be of graded type and 

 becoming effective at different times of development in different 

 alleles or compounds; furthermore, that the insufficient quantity 

 in question spreads across the wing from its base and reaches 



