GENES AND DEVELOPMENT I49 



substances produced by the organizer. The substance concerned with the 

 production of neural tissue (known as the neural evocator) can be isolated 

 in an impure form; its nature is still in doubt, but there are some grounds 

 for suggesting that it may be sterol-Uke.^ It is extremely unspecific, in 

 the sense that it is quite independent, both in origin and in action, of 

 the specific constitution of the tissues involved. Thus frog evocator will 

 work on newt tissue, and chick evocator on mammals. It is even foimd 

 in groups widely separated from the vertebrates; most remarkably of 

 all in hydroids, which do not even possess a nerve cord. 



This suggests that the genetic constitution of an egg acts by modi- 

 fying the reaction to evocators, and not by determining the character 

 of the evocators themselves. Thus when frog evocator aas on newt 

 tissues, it induces newt neural tissues. It seems fairly certain, in fact, 

 that genetic factors control histological type by controlling the reactivity 

 or "competence" and not by controlling the evocators. 



The situation is more complicated when we consider not only the 

 histological type of the induced tissues but also the pattern in which 

 they are arranged. Again most of the experiments have been made 

 between different species or genera, and the genetic analysis is still 

 lacking. It is certain that the pattern in which the induced tissues are 

 arranged is partly dependent on the organizer. The process by which 

 the organizer influences the pattern is called "individuation," and it is 

 probably performed by some method other than the diffusion of a 

 single homogeneous chemical substance; evocator extracts do not 

 transmit any pattern to the reacting tissues. The organizer-pattern is, 

 as might be expected, closely related to the specific nature of the 

 organizer. A large Axolotl organizer induces an Axolotl-sized neural 

 plate in frog ectoderm,^ and similarly for other combinations. 



It is remarkable that organizers from different groups share pattern- 

 properties which may not be at all apparent in normal development. A 

 newt has nothing to correspond to the sucker of a frog tadpole. But if 

 frog ectoderm is grafted into a newt embryo in a position similar to that 

 in which the frog develops a sucker, there is some influence there which 

 induces a sucker to form.^ In this position there must be some similarity 

 between the two organization centres. It is difficult to beUeve, however, 

 that the similarity could be sufficientiy detailed to determine the actual 

 structure and arrangement of the sucker, and it is probable that the 



^ Waddington, Needham, and Brachet 1936, Needham 1936. 



- Holtfreter 1935. 



' Spemann and Schotte 1932, cf. Rotman 1935a, fc, Twitty 1936. 



