THE EPIGENETICS OF THE EMBRYONIC AXIS 207 



ceeding in the nucleus itself, or it may merely mean that amino-acid 

 turnover is more rapid in the nuclear than in the cytoplasmic proteins. 

 As mentioned above, the nuclear uptake begins first in the region of the 

 blastopore, and rapidly spreads into the neural plate. It is probable that 

 in the next few years a great deal of information will become available 

 by this method. 



A third method by which a direct approach has recently been made to 

 the problem of protein synthesis as the basis of determination is the use of 

 immunological techniques. Much of the older work in which such 

 methods were used on embryos (Review: Needham 1942) was con- 

 cerned with the development of species specificity; it is a well-knoMOi, 

 though remarkable, fact that tissues of young embryos of quite different 

 species or genera of Amphibia and birds may be grafted together and 

 prove themselves mutually compatible until late stages in development. 

 Even grafts between early embryos of fish and Amphibia or mammals 

 and birds survive for a period which may be quite considerable. Neverthe- 

 less recent workers have been successful in demonstrating the existence 

 of organ-specific antigenic substances in very early stages of development, 

 and in using immunological methods to study the changes in such sub- 

 stances as development proceeds. References to most of the recent 

 literature will be found in Woerdeman (1953), Cooper (1950) and Clay- 

 ton (1953) for the Amphibia and Ebert (1952) for the chick. 



Most authors have prepared anti-sera by injecting (into rabbits) minces 

 or extracts of tissues taken from adult animals, and have then tested 

 extracts of embryonic organs to discover whether they react with the 

 anti-serum; if they do, it is concluded that they contain an antigen 

 similar to, or the same as, that in the adult organ. However Clayton 

 (1953) and Fhckinger and Nace (1952) prepared their anti-sera by injection 

 of embryos, or parts of them. Clayton made anti-sera against (i) early 

 gastrulae of Triturus alpestris, (2) ectoderm of the early yolk-plug gastrula, 

 (3) archenteron roof of the same stage, and (4) whole tail-buds. When 

 extracts of an embryonic organ were to be tested against these, the whole 

 anti-serum could be used for the test, or it could be first subjected to a 

 process of fractionation by the method of selective absorption ; for in- 

 stance, anti~tail-bud serum can be allowed to react with an extract of 

 gastrula until all the antibodies against antigens present in the gastrula 

 are removed, so that only the antibodies against antigens wliich arise 

 between the gastrula and tail-bud stages are left. Using this method, 

 some important, though still tentative, conclusions were reached; the 

 reason for hesitation in putting them forward as fully proved is that 

 immunological methods, although extremely sensitive, cannot of course 



