A. TYLER 353 



during the first days after birth (Hanan and Oyama, 1954; Dixon 

 and Maurer, 1955; Cinader and Dubert, 1955). The unresponsive- 

 ness persists long after the antigen becomes undetectable in the 

 serum, although possibly persistent in the tissues, and does not 

 affect the ability of the animal to respond to an unrelated antigen. 



Bilhngham et al. ( 1955a,b, 1956 ) suggest that the immunologi- 

 cal paralysis induced by large doses of pneumococcal polysac- 

 charide ( Felton, 1949 ) might be related to the "actively acquired 

 tolerance." However, this seems to involve neutralization of anti- 

 body, as it is formed, by the pneumococcal polysaccharide antigen 

 fixed in the tissues (cf. Dixon and Maurer, 1955), rather than a 

 "central derangement of the antibody-forming capacity" as Bil- 

 lingham et al. describe their results. One possibility that has been 

 suggested (Tyler, 1955b) to account for such a central derange- 

 ment is that the host cells concerned in antibody formation un- 

 dergo a "type-transfonnation" of the sort exhibited by microor- 

 ganisms in response to specific DNA. The experiments with 

 antigens such as serum albumin would rule out a direct action of 

 specific nucleic acid, in the sense of genetic alteration of anti- 

 body-forming cells by DNA. Alternatively one might assume (as 

 suggested on p. 350) that the embryo or newborn manufactures 

 antibodies against foreign antigens but retains them as an integral 

 part of the cell. As such these fixed antibodies might serve as 

 templates (see "The Concept of Natural Auto-Antibodies," be- 

 low) for the formation of antigenic structures of the type origi- 

 nally introduced. If cells of the adult organism have thus acquired 

 a particular antigenic specificity they would not be expected to 

 respond to the same antigen. Other possible interpretations have 

 been discussed by the cited investigators in this field. Since it is 

 still uncertain as to whether or not all the reported experiments 

 represent the same kind of acquired tolerance and since the field 

 is being actively explored, one may expect that current research 

 will soon provide some of the answers. 



In connection with the above-cited work it is of interest to re- 

 call some early experiments of Murphy (1914a,b). He showed 

 that tumor tissue of a rat could grow readily in the avian embryo, 

 but when the foreign implants were made at the time of hatch- 



