THE CATALYST ENTELECHY IN DIFFERENTIATION 187 



become nerve tube, would develop into brain or spinal chord. 

 That is, up to this stage in gastrulation, the regions are undeter- 

 mined and develop in step with their new actual surroundings 

 (ortgemdss), and not according to what their natural undisturbed 

 position would have made of them. 



Gastrulation, however, ends this plasticity and irrevocably de- 

 termines the fate of each part, which can then undergo only its 

 own special type of development, even if grafted on to another 

 embryo. That is, the bit of transplanted tissue now develops ac- 

 cording to its self-contained possibilities and its place of origin 

 (herkunftsgemass), and not according to its new place of growth. 

 Thus an eye region will develop an eye, even though the eye may 

 point inward. As Needham puts it,* the invisible process of 

 determination has ushered in a new period of self-differentiation, 

 in which the embryo has become a mosaic of irreplaceable regions, 

 similar to the development of certain eggs ("mosaic eggs"), which 

 never manifest a period of plasticity or pluripotency. 



But one particular region in the amphibian embryo is much 

 less plastic than others — the dorsal lip of the blastopore which 

 has arisen from the grey crescent and will eventually form the 

 mesoderm, notochord, somites, etc. A piece of this region, when 

 grafted into another embryo in the blastula or early gastrula stage, 

 will cause, direct, or induce the nearby host tissues to form a 

 secondary embryo, including nerve tube, brain, eyes, ears, somites, 

 notochord, etc., irrespective of what might have developed if no 

 transplantation had been made. So Spemann called this fate- 

 determining portion of the blastopore the organizer (in German 

 organisator). 



Later it was found that local developments in already deter- 

 mined regions, e.g., the induction of a lens from the ectoderm 

 by the eye-cup, were dominated by local inductors. Spemann 

 therefore (1924) termed these secondary organizers. Thus, in the 

 case of the eye, if the dorsal ectoderm had not first been stimu- 

 lated by the "grey crescent" region, no neural tube and therefore 

 no eye-cup would have been formed. So the bit of living tissue 

 which heads what Needham terms "the heirachy of organizers in 

 animal development," was called by Spemann the primary organ- 

 izer, while organizers acting at later successive stages are secon- 

 dary, tertiary, etc. (see Figure 30). 



* Needham suggests the following English equivalents: neighborxvise for orts- 

 gemass; selfwise for herkunftsgemass. 



