Genlc Control of Development 329 



never need the stimulus of the inductor but develop just as in mosaic 

 development (e.g., the intestinal tract). Holtfreter then presents some 

 examples of what we considered before as predetermination, and con- 

 cludes (with others) that there must be self-reproducing cytoplasmic 

 elements, the plasmagenes, originally produced by genie (nuclear) 

 action and identical with Claude's microsomes and Brachet's baso- 

 philic granules. 



These concepts are applied to the inductor action and the compe- 

 tence for it. The cells of the germ layers contain cytoplasmic elements 

 which are responsible for the various differentiations, which he calls 

 neurogenes, myogenes, and so on. Now the X-substance (the inductor) 

 is present everywhere, but is inactive. In the experiments it is liberated 

 by cutting the cells; in normal development, by separation from what 

 prevents its function; this may be the neurogene N. The X-substance 

 behaves like an enzyme; it makes more X and N dissociate, and the 

 process spreads to adjacent cells. N is now supposed to be the pre- 

 cursor of a plasmagene; by its liberation from X it "mutates" to the 

 functional neurogene, which is self-propagating, and thus fills all 

 cells derived from the induced one. Holtfreter then proceeds to 

 "explain" competence on the basis of this plasmagene concept, namely, 

 the question why the response of a tissue to the inductor stimulus 

 changes in time and is genetically controlled, while narrowing down 

 its developmental potencies. According to Holtfreter, the young cells 

 contain precursors of many types of plasmagenes, which compete with 

 each other for a substrate needed for the synthesis of a functional 

 plasmagene. An external agent or an inductor substance or a nuclear 

 product selectively induces one of these precursors to begin its syn- 

 thetic activity, and it begins to multiply, taking more and more sub- 

 strate away from the other latent plasmagenes. Finally, the cell be- 

 comes competent only for the one type of plasmagene, which has been 

 selected. 



Thus what we called "genie activation," meaning the specific 

 action of genie material only in the presence of the proper cytoplasmic 

 substratum, is replaced by the rather miraculous doings of plasma- 

 genes. It is done by omitting reference to development without in- 

 ductors, which has all the features of development in common with 

 the inductor type, except for the absence of the inductor. I think that 

 the problem of progressive determination controlled by the genie 

 material must be solved first as such, and that only afterward can the 

 specific features of the intercalation of an inductor be added. If this 

 is done, an explanation of the type presented previously appears 



