126 INDUCTION AND ORGANISATION 



isine" concentration. At very high concentrations, a notochord 

 would be formed; at a lower concentration, somites, and so on. 

 Which differentiation occurs, would depend on whether or not 

 the concentration exceeds a certain liminal value. The regulation 

 phenomena can easily be explained on this basis. After disturb- 

 ances in the germ, the organisine would be redistributed. Simple 

 diffusion would lead to an equilibrium in which its concentration 

 decreased again in all directions from a central maximum. The 

 expansion of the organisation-field from a grafted organiser 

 into the surrounding tissues can also be explained simply by 

 diffusion of organisine, and the same applies to the trans- 

 ference of organising activity to indifferent material grafted 

 into the organisation centre. 



Later, Dalcq has extended his hypothesis by attributing 

 great importance to the ''physiological competition'' between 

 parts of the germ. This concept was introduced by Spiegelman 

 (1945), who used it in a theoretical discussion of the phenomena 

 of regeneration (cf. Ch. XI). Neighbouring groups of developing 

 cells are supposed to compete by monopolizing food substances, 

 and by flooding each other with the waste products of their 

 metabolism. A cell group favoured in some way, would thus 

 be able to suppress the differentiation of other groups in its 

 environment. This is the so-called phenomenon of dominance. 

 Dalcq (1947) applied this idea to the relationships in the 

 organisation-field, and assumed that the cells compete for the 

 limited quantity of "organisine" that is available. He succeeded, 

 indeed, in giving simple explanations of several observed 

 phenomena on this basis. 



Dalcq's expositions form a fertile working hypothesis which 

 deserves to be tested by further experiments. Several recent 

 studies have already given it strong support, e.g. the work 

 of Raven and Kloos (1945) who investigated the inductive 

 powers of the median and lateral parts of the archenteron 

 roof. In any case, the hypothesis shows that it is possible, in 

 principle, to tackle the problem of organisation, however 

 mysterious it may seem at first sight, and to find a simple 

 explanation for the origin of the orderly spatial multiplicity 

 of the germ. 



