lo PATTERNS AND PROBLEMS OF DEVELOPMENT 



that more or less differentiation of neural tissue in amphibian embryos 

 could be induced not only by the living tissue of the dorsal lip region but 

 also by various other amphibian tissues of the same or other species, gen- 

 era, or orders; and not only that, but dead tissues, as well as living, proved 

 to be inductors. With further experiment many living and dead tissues 

 from many different animals, representing most invertebrate and verte- 

 brate groups, tissue extracts, and various synthetic chemical substances, 

 proved to have more or less inducing action. Some of these foreign in- 

 ductors are apparently organizers: certain plant tissues, for example, have 

 been reported to determine neural plate. Induction is not restricted to 

 amphibian development but occurs in other vertebrate embryos; and in 

 hydroids and planarians small pieces (in planarians chiefly pieces from 

 the region of the cephalic ganglia) may determine new axiate patterns (see 

 pp. 378-87). In the hydroid, Corymorpha, a lacerated incision may serve 

 as an organizer.^ Moreover, photic, electric, thermal, and chemical dif- 

 ferentials and gravity can determine new physiological axes and develop- 

 mental patterns in at least some plant or animal species. These agents are 

 as truly inductors and organizers as living or dead tissues or organic chem- 

 icals.' Inductors and organizers are, then, nothing new but are simply 

 cases of physiological dominance established in one way or another and 

 are characteristic of development in general. The character of the domi- 

 nance remains to be determined in each case. It is probable that it will 

 be found impossible to make any sharp distinction between inductors and 

 organizers, for, as will appear in later chapters, it may be questioned wheth- 

 er the organizer does anything more than determine or play a part in de- 

 termining a relation of dominance and subordination which becomes the 

 real organizing factor. In view of this possibility, which is supported by 

 experimental evidence, it is perhaps desirable to drop entirely the term 

 "organizer." 



It has usually been assumed that chemical substances are the actual 

 inducing agents in vertebrate development. Whether or to what extent 

 this is the case is perhaps still uncertain as regards some inductions. There 

 is, however, considerable evidence which makes it probable that develop- 



■ For earlier experiments showing or indicating dominance and induction in reconstitution 

 see: Browne, 1909; Rand, 1911, 1912; Child, 1911U, c. Many of the experiments of Driesch, 

 Morgan, Child, and others on the hydroid, Tiibitluria, suggested dominance of the hydranth 

 region and of distal over proximal regions. Dominance of the vegetative tip in plants has long 

 been familiar to botanists. 



' Experimental determination of axes, inductors, and organizers will be more fully discussed 

 in chaps, xi and xii. 



