154 organisers: inducers of differentiation 



the organiser. If three extra organisers are grafted into the close 

 vicinity of an organiser in an intact embryo so that their polarities 

 all converge to a point in the centre of the host-organiser, there is 

 no inductive effect of any kind.^ This annihilation of the inductive 

 effect is difficult to understand. It can scarcely be that an intact 

 structure, or an unimpeded gastrulation-process, are essential pre- 

 liminaries to the production of the chemical substance responsible 

 for the organising effect; for even if a piece of the organiser is 

 made to wait for some time before it is grafted, when it rolls up into 

 a ball, and the arrangement of its cells is markedly altered, its 

 organising power is not affected 'or reduced.^ 



The possibility that the organiser effect in birds is in some way 

 dependent on the normal tissue-movements which take place in 

 gastrulation, i.e. on so-called "dynamic determination " (Vogt), will 

 be discussed below (pp. 163, 250). 



Recently, the decisive discovery has been made that cell-free 

 fractions of a liquid extract of whole neurulae can exert an organising 

 action, as evidenced by neural tube induction. The liquid is coagu- 

 lated by heat and portions of the resultant solid material implanted 

 into the blastocoele. The active substance is certainly ether- 

 soluble, and probably lipoidal.^ 



Meanwhile, some interesting results have emerged from investi- 

 gations into the glycogen-content of the cells of the amphibian 

 embryo. This is high in the cells of the animal hemisphere ; low in 

 those of the vegetative hemisphere, and intermediate around the 

 equator. But as soon as the cells of the organiser have become in- 

 vaginated, they immediately lose what glycogen they contained. It 

 is not improbable that this sudden disappearance of glycogen con- 

 notes an expenditure of energy connected with the physiological 

 activities characteristic of the organiser.* 



§4 



The fact that the organiser, in the form of the primitive gut-roof, 

 is capable of organising the epidermis overlying it so as to induce 

 it to give rise to neural folds, explains a number of phenomena 



^ Goerttler, 193 1. ^ Holtfreter, 1933 b. 



^ Waddington, Needham and Needham, 1933. 

 * Woerdeman, 1933 a; Raven, 1933 b. 



