organisers: inducers of differentiation 145 



the newt show that its site is already determined and locaUsed ten 

 minutes after fertiUsation.^ Rather later, portions of the blastula 

 in the region of the grey crescent have been found to possess 

 the inductive property.^ As to the time at which this property 

 is lost, it has been shown that the notochord, which of course 

 is formed from the invaginated organiser, retains for a consider- 

 able period the power of inducing the formation of neural 

 folds.3 



It has also been shown that in the neurula stage, myotome material, 

 which of course w^as originally derived from the organiser region, 

 still retains the capacity of inducing neural tube formation from 

 presumptive epidermis when grafted into an early gastrula. How- 

 ever, slightly more lateral mesoderm material, which had dif- 

 ferentiated into pronephros, in similar experiments only induced 

 other pronephric tubules.* 



This is known as '* homoiogenetic induction", to contrast it with 

 the heterogenetic power of the organiser, which induces the forma- 

 tion of structures different from itself. It is found that the neural 

 plate, once underlain by the organiser, possesses and retains for 

 a very long time — certainly up to the free-swimming larva — this 

 power of inducing the formation of structures of its own type. This 

 is proved by grafting portions of neural tube into blastulae, where 

 secondary neural folds are induced.^ It is of interest that the hind- 

 most portion of the neural fold region of the neurula induces the 

 formation of mesoderm, which agrees with the fact that this region 

 gives rise to the muscles of the tail in normal development (Chap. 11, 

 p. 28).® Accordingly, this induction also is homoiogenetic. Lens 

 rudiments implanted into blastulae have no power of induction, 

 either hetero- or homoio-genetic.^ 



Spatially, the region of the blastula and early gastrula which has 

 organising capacities appears to coincide with the region which will 

 become invaginated at gastrulation, i.e. the presumptive notochord 

 and axial mesoderm regions.^ 



This is a large area, and it might be expected that there would be 



^ Fankhauser, 1930. 2 Bautzmann, 1926. 



^ Bautzmann, 1928, 1929 A. * Holtfreter, 1933 b. 



^ Mangold and Spemann, 1927; Mangold, 1929 b. 

 •^ Bytinski-Salz, 193 1. ' Kruger, 1930. 



^ Bautzmann, 1926. 



HEE 10 



