A. TYLER 345 



changes occur in extractable and cellular antigens during devel- 

 opment. For reference to investigations in this field during the 

 past decade may be cited the work on frogs by Cooper (1946, 

 1948, 1950), Ten Gate and Van Doorenmaalen (1950), Flick- 

 inger and Nace (1952), and Spar (1953); on salamanders by 

 Woerdeman (1950, 1953a,b) and Clayton (1953); on birds by 

 Schechtman (1947, 1948, 1952, 1955), Nace and Schechtman 

 (1948), Briles, McGibbon, and Irwin (1948; cf. Ii-win, 1949, 

 1951), Schechtman and Nace (1950), Ten Cate and Van Dooren- 

 maalen (1950), Schechtman and Hoffman ( 1952 ) , Ebert (1950, 

 1951, 1952, 1953, 1954, 1955), Miller (1953), and Nace (1953); 

 on mammals by Maculla (1948), Yeas (1949), Chernoff (1953), 

 and Goodman and Campbell (1953); on sea urchins by Perlmann 

 and Gustafson (1948), Perlmann (1953), and Harding, Harding, 

 and Perlmann ( 1954 ) ; and on silkworms by Telfer and Williams 

 ( 1953 ) and Telfer ( 1954 ) . Most of this work has been recently 

 reviewed (see Ebert, 1955; Nace, 1955; Tyler, 1955b) and the 

 present discussion will be limited to a brief consideration of cer- 

 tain special features. 



One must recognize in the first place that technically it is not 

 possible at present to prove the complete absence of an antigen; 

 that failure to detect an antigen does not prove its absence. Thus 

 Ten Cate and Van Doorenmaalen ( 1950 ) detected lens antigen 

 at earlier stages of development than had Burke et al. (1944). 

 They attribute this to the use of more sensitive procedures. This 

 suggests that the antigen might be detected at still earlier stages 

 of development if further technical sensitivity were achieved. 

 The concentration of lens antigen evidently decreases the earlier 

 the embryonic stage tested, but there is no clear justification for 

 the assumption that it extrapolates to zero. The same considera- 

 tions apply to the interpretation of the various experiments cited 

 above that deal with other organ-specific antigens, such as those 

 of brain, spleen, kidney, and heart. In general, then, the question 

 of preformation in embryonic development is raised again, but 

 in a manner that does not permit easy resolution. Many antigens, 

 such as those of vertebrate red cells that characterize blood 

 groups and blood types, are known to be gene determined. It is 



