PROVOCATIVE BIOLOGICAL THEORIES 79 



And again he found his conclusions substantiated by his 

 experiments, for not only did this transplanted portion of 

 the dorsal lip cause a similar growth in an animal of 

 another species, but strangest of all, the new "secondary 

 embryo" was made up of the tissue of the animal to which 

 the graft had been transplanted and not of the tissue of 

 the graft. 



The grafted portion of the dorsal lip, therefore, is 

 thought of as an inducer of growth and differentiation of 

 the tissues in which it is placed or among which it is 

 found. This differentiation is supposed to be determined 

 by and controlled by enzymelike secretions, not so much 

 from the individual cells as from certain groups of cells. 

 It is to these groups that the name of Organisationcentra 

 has been given. 



The Axial Gradient Theory. It seems that the Ger- 

 manic peoples have been more influenced by the Organi- 

 sationcentrum idea, while biological workers in England 

 and a few Americans have found in Child's Axial Gradi- 

 ent Theory a source of new paths down which to direct 

 their experimental energies. 



The hold that Child's theory (1915) has taken on some 

 workers, may be gauged by the fact that Professor F. W. 

 Gamble, in his presidential address before the zoological 

 section of the British Association for the Advancement 

 of Science in 1924, not only made this theory the main 

 thesis of his address, but held that "no more fertilizing 

 biological idea has been disseminated in the last ten years 



