APPENDIX 497 



the cleavage of Nereis may be regarded as organ-forming substances 

 rather than raw materials (see p. 217): further experiments, in- 

 volving isolation of blastomeres and centrifugalisation, combined 

 with study of /)H indicators, will have to decide on this point. As 

 an example, however, of the stratification of substances (perhaps, 

 of potencies, see p. 102) resulting from the action of an electrical 

 gradient, these observations and experiments are of great value. 



4. Organiser-properties in living and dead tissues. 



In amplification of the statement on p. 153 that certain tissues 

 which possess no capacity to act as organisers when alive may 

 show this capacity when they are killed, we may refer to further 

 recent experiments by Holtfreter (N aturwissenschafteyi^ xxi, 1933, 

 p. 766). He has found that the property to induce the formation 

 of a secondary embryo or parts of it in an amphibian gastrula 

 are possessed by the following : all parts of uncleaved amphibian 

 eggs that have been boiled to a state of hardness ; all parts of an 

 amphibian gastrula that have been preserved for six months in 

 70 per cent, alcohol, treated with xylol, embedded in paraffin and 

 brought back to water; boiled pieces of muscle of the Annelid 

 Enchytraea and of the molluscs Planorbis and Limnea\ heat- 

 coagulated cell-free extracts of the crustacean DapJmia and of the 

 pupae of moths ; pieces of all organs so far tested of the stickle- 

 back, fresh or boiled; living pieces of larval amphibian liver, 

 brain and retina, and of adult liver, ovary, and heart ; living pieces 

 of liver, kidney, testis and other organs of lizards, birds, and mice ; 

 coagulated bird embryo-extract ; extract of killed calf's liver, and 

 boiled pieces of several mammalian organs; pieces of liver, brain, 

 kidney, thyroid and tongue of a fresh human corpse. 



No vegetable material was found to possess organising pro- 

 perties, but in the animal kingdom the chemical substance which 

 forms the basis of the organising action is clearly widespread and 

 may probably be regarded as universal. The fact that it is absent 

 (in the living state) from all regions of the vertebrate embryo 

 except the organiser, while it is present in a variety of organs in 

 the adult, is noteworthy and is probably to be regarded as an 

 adaptation to a specialised mode of development in which organ- 

 iser action by contact (p. 310) is employed. 



HEE 32 



