CHAPTER Vm 



Induction and organisation 

 L Neurulation 



The cell migrations during gastrulation, and the further topo- 

 genetic processes to which the cells are subjected, bring the 

 materials for the embryo's future organs into those places 

 where they belong according to its structural plan. But they 

 also have another consequence. Cell groups which originally 

 were separated by considerable distances, and which, as a 

 result of chemodifferentiation, differ in physical and chemical 

 condition, now become immediate neighbours. It may be ex- 

 pected, therefore, that they will now influence one another, and 

 that the new topographical relations between the cell groups 

 will in this way initiate new processes of chemodifferentiation. 

 The outcome of these topogenetic processes will thus be a 

 considerable increase in the spatial multiplicity of the embryo. 

 There are, indeed, some observations, which give clear 

 evidence of changes in the physico-chemical constitution of the 

 cells during gastrulation. Woerdeman (1933) was the first to 

 show that a sudden fall occurs in the glycogen content of the 

 glycogen-rich marginal zone cells of amphibians, at the moment 

 when these cells are rolled in over the lip of the blastopore. 

 The following experiments proved that there was a causal 

 connection between the processes of invagination and of 

 glycogen decomposition. First of all, dorsal marginal zone 

 material of an early gastrula was grafted into the ventral margi- 

 nal zone of another germ. It then became rolled in, either over the 

 ventral lip of the host blastopore, or over the edge of a second- 

 ary blastopore (see p. 101). The moment of disappearance of 

 the glycogen always coincided with that of rolling in. Secondly. 



