APPENDIX 493 



i. So far as it goes, there is in the exogastrulation experi- 

 ments a determination for the presumptive neural fold region 

 to stretch more than the neighbouring presumptive epidermis 

 (p. 484), and this determination must be independent of an 

 invaginated organiser. 



ii. On comparing the experiments in which the organiser-region 

 is removed or inactivated with the exogastrulation experiments, it 

 will be noticed that there is a difference in the distance between the 

 presumptive neural fold region and the organiser, and in the time 

 during which the latter could act on the former at any given distance. 

 While previous experiments have led to the view that there exists 

 a gradient-field under the influence of the organiser before in- 

 vagination, in which the presumptive neural fold region undergoes 

 labile determination, the conditions of the exogastrulation experi- 

 ment are such that it might be argued that the gradient-field is 

 deformed, or even not formed. 



iii. The labile determination in question would be part of the 

 general effect of an uninvaginated organiser working from a 

 distance as the centre of an individuation-field (see p. 310). It is 

 held by Holtfreter that neural fold formation can result only by 

 contact with an underlying organiser ("evocation", Waddington 

 and Needham). But it is clear from the experiments on newts (see 

 p. 149), in which trunk-organiser is grafted at head-level and 

 induces the formation of head-structures, that the tissues there are 

 under the influence of an action exerted from a distance by the 

 host-organiser acting as the centre of an individuation-field. The 

 same conclusions emerge from experiments on birds (see p. 162). 

 There is therefore evidence for the existence of individuation-fields, 

 in both Amphibia and birds, which is not disproved by the exo- 

 gastrulation experiment. 



iv. The variations between the results of explantation of portions 

 of blastulae in inorganic media, and those of interplantation of 

 similar portions into living embryos (see pp. 139, 317), show that 

 the reactivity of the tissues (i.e. their differentiation into epidermis, 

 neural tube or notochord) is markedly affected by environmental 

 changes. It is possible that the lack of differentiation of neural 

 structures in the exogastrulation experiments is to be explained on 

 such lines as these. 



