THE EPIGENETICS OF THE EMBRYONIC AXIS 2I5 



clear whether these substances are in the form of individual aniino-acids 

 or of larger, more complex particles. There is some evidence that, at later 

 stages, after the blood circulation is established, different organs may give 

 off substances v^hich are complex enough to carry tissue specificity. Thus 

 Ebert (1953) reports that if grafts are made on to the chorio-allantoic 

 membrane of the chick, with fragments of spleen, liver or kidney from 

 adult fowls injected with methionine-S^^, the radioactivity is found later 

 to be specifically accumulated in the embryonic organ corresponding to 

 the graft; that from spleen grafts goes primarily to the embryonic spleen, 

 from kidney to the embryonic kidney, and so on. However, Sirlin and 

 Waddington (1955) were not able to find any evidence of such organ- 

 specific transfers of substances in the early stages of the amphibia, before > 

 the onset of circulation. They could also not confirm Ficq's observations 

 of a passage of the tracer from a labelled organiser into the host neural 

 tissue as well as into the induction. They point out that the tracer which 

 gets into the induced tissues is mainly in the nuclei, and therefore gives no 

 evidence for a passage of cytoplasmic microsomes. They suggest, indeed, 

 that the tracer may actually be passing in the form of the free amino- 

 acid, in which case it will yield httle information about the mechanisms 

 of induction. 



Abercrombie and Causey (1950) have used radio-phosphorus to label 

 regions of the cliick primitive streak which were then used as inducing 

 grafts into other blastoderms. They were able to distinguish the graft 

 tissues fairly sharply from those of the host but their technique was not 

 adequate to decide whether any minor spread of labelled compounds 

 from the graft had taken place. 



Thus the experiments using radioactive labelling have not yet given 

 unequivocal evidence that bodies as large as microsomes pass from the 

 mesoderm into the ectoderm during evocation, but neither do they refute 

 the suggestion. 



6. Regionally specific evocation 



As was mentioned earlier, Spemann and many later authors have shovwi 

 that the first invaginated, presumptively anterior, part of the mesoderm 

 has a strong tendency to induce the anterior part of the nervous system, 

 while the presumptively posterior mesoderm tends rather to induce trunk 

 or tail regions. These phenomena present us with two rather different 

 types of problem. On the one hand there is the question of whether the 

 anterior and posterior parts of the organiser owe their specific effects to 

 chemical differences in the evocators which they release. This is important 

 for our present discussion, since if it were true it would be good evidence 



