THE MIT ATED GENE 181 



from the surroundings. But since Ephrussi (1935) has shown 

 that tissue containing homozygous lethals may be grown in 

 tissue culture, it might also be possible that an influx of some 

 product controlled by the normal chromosomes is not involved 

 in Baltzer's case but rather a draining off of poisonous products of 

 abnormal metabolism. In agreement with this interpretation 

 is also the fact that Hadorn (1934) was able to grow merogonic 

 cells in tissue culture. 



B. Reactions of an Inductive Type 



The experiments just mentioned lead to the type of embryologi- 

 cal process called induction. A number of facts are known that 

 prove that the products of gene-controlled reactions are formed 

 in definite areas of the embryo at a definite time and spread 

 hence into neighboring territories where they induce certain 

 morphogenetic processes. These facts constitute the most direct 

 link between genetics and embryology. 



1. Terminology. — When Goldschmidt (19206) tried to derive 

 general ideas on gene action from his experiments in intersexuality, 

 he had to face the following situation. It was proved that sex 

 was controlled in Lymantria by male and female reactions of 

 definite velocity which are timed in such a way that one or the 

 other passes the threshold. Since all the facts of gynandro- 

 morphism seemed to prove that insect development is of the 

 completely self-determinative type, it had to be assumed that the 

 sex-controlling reactions take place within each cell, producing 

 sex-determining stuffs. It was also known that in vertebrates, 

 morphogenesis of sex differentiation is controlled largely by sex 

 hormones; hence it was concluded that these intracellular sex- 

 determining stuffs are physiologically identical with the sex 

 hormones. The difference would be only the production within 

 all body cells versus production in a centralized organ, the gonad. 

 From this, the generalization was derived that all genes produce 

 substances of morphogenetic action similar to hormones, and, 

 therefore, the general term hormones was amplified in its scope 

 to cover also such intracellular morphogenetic stuffs. When 

 later Witschi (1929) proved that in vertebrates a special sex- 

 determining action was exercised by cortex and medulla of the 

 gonads whence it spread by diffusion, Goldschmidt (1931c?) 

 proposed to distinguish three types of morphogenetic substances 



