THE EPIGENETICS OF THE EMBRYONIC AXIS 2I7 



plants nearly always contain mixtures of these substances, one or other 

 predominating in particular cases; it is possible, for instance, to reveal 

 the presence of small amounts of the posterior-inducing substance in 

 guinea-pig liver tissue when the brain-inducing material is removed by 

 thorough extraction with petrol ether. 



The existence of these two different evocators seems rather thoroughly 

 estabhshed by Toivonen's data. There are however several questions 

 about them wliich still remain open. In the first place, should they really 

 be regarded as specific inductors of particular regions, rather than of 

 particular tissues ? It is noteworthy that the archencephalon is the part of 

 the nervous system which, in the normal embryo, is not accompanied by 

 any mesoderm : and when posterior parts of the neural system are induced, 

 some induced mesodermal structures always appear with them. One 

 might in fact suggest (cf. Waddington 1952c) that the archencephahc 

 evocator is a pure neural inductor while the spinal inductor is able to 

 induce mesoderm as well as neural tissue, the anterior or posterior charac- 

 ter of the induced organ depending on the proportion between the 

 amounts of induced neural and mesodermal material. Toivonen (1953) 

 has in fact recently found that alcohol-fixed bone marrow is a specifically 

 mesodermal inductor causing the appearance of induced muscles, extremi- 

 ties, etc., unaccompanied by any induced neural tissue. 



Another problem is the relation between these evocators from adult 

 tissues and the factors active in the invaginating mesoderm of the gastrula. 

 It has been known since the early work of Holtfreter (193 4^) that the 

 organiser material of the gastrula, when extracted with boiHng water, 

 alcohol, etc., retains its power to induce neural tissue but loses that to 

 induce mesoderm. Barth and Graff (1943) showed that the same is true 

 if the organiser region is freeze-dried. Waddington (1952c) re-examined 

 the effect of sHght heat treatment. The abohtion of mesoderm-inducing 

 capacity was confirmed and the evidence suggested that the neural- 

 inducing capacity is not in any way regionally differentiated. Large masses 

 of induced neural tissue sometimes form themselves into archencephahc 

 vesicles (i.e. forebrain) ; and thus it seems not unreasonable to suppose 

 that the heat-resistant neural evocator of the gastrula mesoderm is the 

 same as the archencephahc evocator isolated by Toivonen from adult 

 tissues. Lalher (1950) claims that the neural-inducing capacity of the 

 gastrula organiser is abolished by treatment with formahn, but it is not 

 clear whether this is true of Toivonen's archencephahc evocator. The 

 mesoderm-inducing capacity of the hving gastrula organiser also shows 

 properties not unlike those of the spinal inducer postulated by Toivonen. 

 In particular they are both heat labile. We shall see later when discussing 



