A. TYLER 349 



In addition to the studies with purified proteins there have 

 been a number (e.g., FHckinger and Nace, 1952; Spar, 1953; 

 Clayton, 1953 ) in which antisera produced against sahne extracts 

 of early embryos have been used, after appropriate absorptions, 

 to detect changes in antigenic composition during early devel- 

 opment. Different stages of development and even different em- 

 bryonic germ layers have thus been shown to provide extracts in 

 which a different assortment of antigens is detectable, new anti- 

 gens becoming detectable at certain stages of development and 

 some apparently disappearing. In these, as well as in the experi- 

 ments with antisera against highly purified adult substances 

 tested against saline extracts of early embryo, there is the uncer- 

 tainty as to whether or not a particular antigenic structure re- 

 mains associated with a saline-soluble constituent, and as to 

 whether or not it is available for reaction in specific absorption 

 procedures. In studies with cellular antigens, such as those of the 

 red blood cells (e.g., Briles et al, 1948; Miller, 1953; Yeas, 1949; 

 cf. Irwin, 1949, 1951; Levine, 1948), the analogous question is to 

 what extent the change in surface antigen that is detected during 

 development represents a change in location rather than new 

 synthesis. 



For the immediate future the main utility of studies of the type 

 discussed above would appear to be that they illustrate how im- 

 munological procedures may be employed to demonstrate specific 

 chemical changes in an embryonic tissue prior to its actual dif- 

 ferentiation. Further studies of this type, it may be hoped, will 

 shed some light on the question of whether or not this approach 

 will provide pertinent chemical information of some primary 

 change occuiTing in embryonic tissues during the otherwise in- 

 visible process of "determination." 



Antibody Formation 



Many early investigators have demonstrated that the early 

 embryo, the fetus, and even the newborn animal lack the capac- 

 ity to produce antibodies ( detectable .in the serum ) in response 

 to the injection of a foreign antigen (see reviews by Needham, 

 1942; Beveridge and Burnet, 1946; Burnet and Fenner, 1949; 



