Chapter VI 



ORGANISERS: INDUCERS OF DIFFERENTIATION 



§1 

 The remarkable organising properties of the dorsal Up of the blas- 

 topore of amphibian embryos were discovered in the following 

 manner. In the experiments with newts' eggs of grafting pieces 

 of the presumptive neural fold region into other positions, in order 

 to discover the time at which they became irrevocably determined 

 to develop by self- differentiation, it was observed that the deter- 

 mination of the posterior part of the presumptive neural fold region 

 (i.e. that portion which lies near the dorsal lip of the blastopore) 

 was effected sooner than that of the anterior part (i.e. farther away 

 from the dorsal lip). It looked as if some agency emanated from 

 the dorsal lip of the blastopore like a "flow of determination", and 

 either streamed or was carried forwards^ (see also p. 173). 



This suspicion was confirmed when it was found that if the 

 animal hemisphere is cut off from an early gastrula of the newt, 

 rotated through any angle about the egg-axis, and then stuck on to 

 the vegetative hemisphere again, the neural folds arise in line with 

 the dorsal lip of the blastopore, which, of course, is situated in the 

 vegetative hemisphere. The neural folds therefore arise from tissue 

 which would normally not have formed them, and neural folds are 

 not formed from the presumptive neural fold material which has 

 been rotated away from the meridian of the dorsal lip of the blasto- 

 pore.2 Something of the nature of what Herbst (1901) called a 

 "formative stimulus" appears thus to be associated with the dorsal 

 lip of the blastopore. 



As to the time when the dorsal lip region exerts its organising 

 action, there are two possibilities. The first is to imagine a trans- 

 mission of stimuli through the tissues from the region of the organ- 

 iser before gastrulation ; the second possibility is to attribute its 

 action to the transmission of stimuli from underneath the surface 



1 Spemann, 1916. ^ Spemann, 1906B, 1918. 



