152 organisers: inducers of differentiation 



The facts also permit of the interpretation that the quantitative 

 potency of inductive capacity falls off in a graded way from the 

 dorsal lip region, although this gradient appears to be steep. ^ 

 In birds (p. 161) there appears to be a definite gradation of in- 

 ductive potency along the organiser (primitive streak), this being 

 highest anteriorly and lowest posteriorly. 



The properties of the organiser are not intimately associated with 

 any particular type of cell. If ordinary presumptive epidermis is 

 grafted into the region of the organiser before gastrulation has 

 started, it becomes '* infected" with the power to organise. This 

 has been proved by heteroplastic grafting of a piece of epidermis 

 from Tritofi cristatiis into the organiser region of T. taeniatus. Such 

 a piece of tissue, originally presumptive epidermis, treated in this 

 way, is found when grafted into another embryo to possess all the 

 qualities of a normal organiser." 



Thus, the properties of the organiser seem to be attached to a 

 certain region of the embryo, regardless of the identity of the cells 

 which occupy it. This region, which owes its localisation to the egg- 

 axis and the plane of bilateral symmetry, must be determined in 

 the outermost or cortical layer of cytoplasm of the ^gg. For even 

 when an tgg is forcibly inverted and its contents stream about in- 

 side, the dorsal lip of the blastopore appears in the region of thq 

 grey crescent, i.e. where it would normally have appeared on the 

 surface of the egg.^ Since, however, the cells of this region divide 

 more rapidly (see p. 39), it seems that some physiological activity 

 is set up in this region of the cortex which later affects the dividing 

 cells of this region, to a considerable depth below the surface. 



In passing, it is of interest to note that in certain experiments, 

 e.g. those in which myotome and pronephros material from a 

 neurula were grafted into an early gastrula (p. 145), and those 

 referred to on p. 191, show that the morphogenesis of artificially 

 induced structures may differ considerably from that shown by the 

 same structures in normal development. Thus the epidermis may 

 be induced to form pronephric tubules without passing through 

 a nephrotome-like stage (see p. 32) : portions of brain-like structures 

 may be induced to form from the epidermis by thickening and 

 subsequent delamination without the formation of neural folds.* A 



^ Bautzmann, 1926, 1933. ^ Spemann and Geinitz, 1927. 



^ Weigmann, 1927. * Holtfreter, 1933 b. 



