402 Action of the Genetic Material 



pleiotropic damage. I refer to the discussion of the subject in the 

 section on pleiotropy, and especially the doubts expressed there that 

 this specific t\pe of pleiotropy contains much information on genie 

 action, except that a mutant ( or a deficiency ) acts at a certain time in 

 a detrimental way upon something, for example, neural fluid pressure 

 (Bonnevie), differentiation of cartilage ( Griineberg ) , differentiation 

 of mesoderm (Poulson). What the genie material does in the normal 

 case, however, remains unknown. 



d. Short cuts: inductors, hormones 



In my old (1920a) formulation, genie action was explained as the 

 production or catalysis of chains of reaction leading to the accumula- 

 tion in quantities up to a threshold of an active substance which was 

 called, in an enlarged meaning, a hormone {Genwirkstoff, or deter- 

 mining stuff, including specific enzymes ) . It is a very remarkable fact 

 that, in the course of phylogenesis, the genie material has developed 

 some kinds of short cuts. In the system of genie actions as it works in 

 t)-pical mosaic development, for example, in Drosophila, it is probable 

 that the genie products are sjTithesized in each competent cell and 

 either do not diffuse or do so ver\- Little (barring a few exceptions 

 found by Sturtevant, \\liiting, and Hannah; see Hannah, 1953). It 

 seems that evolution has led to the establishment of a simplification 

 of this procedure of determining embryonic parts cell by cell by 

 delegating the determining power to diffusible substances of different 

 kinds, inductors and genuine hormones. The first steps have already 

 been made in some groups of insects other than those containing the 

 most popular materials for genetic investigation, Diptera and Lepi- 

 doptera (see Seidel, 1936). In libeUuHds, for example, a formative 

 center is established in the posterior region of the egg. The seg- 

 mentation nuclei must be brought in contact with this center in order 

 to start development. Obviously, some kind of inductor substance is 

 produced there and diffuses to the sites of embr\'onic development. 

 This comes very near to the vertebrate inductor, Spemann's organizer 

 (see III 5 A h), of which nothing is left now but Holtfreter's 

 X-substance. It turned out that the inductor substance is not a specific 

 formative substance but a kind of generalized helper in the establish- 

 ment of the major regions of the embryo. It is exactly what we just 

 called a short cut in the control of development. 



At this point a few words should be added about an exceptional 

 case which shows a secondary genie action of a strange type. It is 

 known that spotting in guinea pigs does not behave in a regular way, 



