NEURULATION 



125 



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1938 a), (Fig. 46). This means that here we have another case 

 of assimilative induction; the piece of ectoderm has acquired 

 organiser properties by its stay in the organisation-field; this 

 is the reason why it is able to give rise to a new organisation- 

 field after transplantation into an indifferent area. It is possible, 

 in other words, to transfer the organ- 

 ising power to indifferent material. This 

 is fresh evidence of the freedom of the 

 field with regard to the material ele- 

 ments of the germ. 



At first sight, it may seem that the 

 concept "organisation-field" does not 

 help us much to acquire a real under- 

 standing of the phenomenon of organ- 

 isation. It does not reduce the great 

 problem, how qualitatively different 

 organs can arise side by side from one 

 common material. The concept, however, 

 has shown its fecundity by giving rise 

 to working hypotheses for the solution 

 of this problem. Dalcq's hypothesis may 

 be taken as an example. Its main features 





i 



are that the organisation-field is sup- 



Fig. 46. The ecto- 

 derm graft has ac- 

 quired the properties 

 of an organiser be- 

 cause of its stay in 

 the blastopore lip. It 

 has organised the 

 primordium of a se- 

 condary embryo in the 

 right flank of the 

 host, o : secondary 

 blastopore. 



posed to be a gradient-field (cf. p. 42), 

 due to local differences in the concentra- 

 tion of a substance, ''organisine", iden- 

 tical with that which is responsible for 

 contact induction. Its concentration 

 would be highest in the centre of the 

 dorsal marginal zone, and from there it would decrease in all 

 directions, though perhaps not everywhere at the same rate. 

 This chemical gradient-field, in turn, is supposed to owe its 

 origin to the interaction of the yolk gradient and the cortical 

 gradient of the egg (cf. pp. 49 and 105). Here, however, it 

 would be the ratio, and not the product, of the factors C and V 

 that would be important (cf. Fig. 38). The origin of qualitatively 

 different organs in an orderly spatial arrangement would be 

 due to the dependence of cell differentiation upon the "organ- 



