organisers: inducers of differentiation 153 



corresponding set of facts is known from the study of normal 

 events in Ascidians, where the same organ may be formed by quite 

 different morphogenetic processes and sometimes even from dif- 

 ferent germ-layers, in development from the egg and development 

 by budding. Similar cases are also known in regeneration. 



§3 



Concerning the physico-chemical aspect of the method of action 

 of the organiser, little can be said, although the results so far ob- 

 tained are of the greatest interest. In the first place, it is clear that 

 the inducing tissue does not require to be alive in order to exert its 

 effects. After an organiser has been subjected to a narcotic (tri- 

 chlorbutyl alcohol) for a certain length of time, the tissues of the 

 organiser may be so heavily damaged that they disintegrate after 

 being grafted, but a secondary embryo is nevertheless induced.^ 



Even more drastic treatment, such as desiccation, or killing with 

 high temperatures, or immersion for 3I minutes in 96 per cent, 

 alcohol, does not destroy the inductive capacity of the amphibian 

 organiser region.^ (See also p. 497.) 



It would seem therefore that the inductive effects of the organiser 

 are due to some chemical substance which is elaborated by it, and 

 support for this view is provided by the fact that pieces of agar jelly, 

 or of gelatine, after being in contact with inductive tissue (neural 

 folds) are themselves capable of inducing.^ 



The question next arises as to whether the initiation of the in- 

 ducing effect, and therefore the productionof the necessary chemical 

 substance, is in any way dependent on the intimate structure of 



^ Marx, 1930. 



^ Here a new complication is introduced by the fact that certain tissues which 

 possess no inductive capacities when alive, such as epidermis and endoderm, 

 are able to act a organisers when killed. While the detailed significance of this 

 fact is still obscure, it is of interest in suggesting that the normal living 

 organiser differs only in some physical degree, and not in kind, from the tissues 

 of the remainder of the embryo. (Spemann, 1929; Bautzmann. Holtfreter, 

 Spemann and Mangold, 1932; Holtfreter, 1933c.) 



It may here be noted that living regenerating amphibian tissue (adult newts 

 12-day limb regeneration-buds) is capable of inducing neural folds in blastulae 

 of the same species when introduced into the blastocoel (Umanski, 1932 b). 

 Similar results have been obtained with insertions of mammalian and avian 

 malignant tumour tissues (Woerdeman, 1933 c). No control experiments have 

 yet been made with non-malignant tissues of the same species. 



^ Bautzmann, Holtfreter, Spemann and Mangold, 1932. 



