26 JOHN H. NORTHROP 



The properties of viruses outlined above may be accounted for in 

 terms of the present hypothesis, as follows : 



Specificity of Virus and Host. Viruses in general increase only in 

 related host species. It is known from immunological studies that the 

 proteins of related species are similar and some may be identical. 



It would be expected, therefore, that the virus precursor (protein- 

 ogen) would be similar or identical in related species but different in 

 widely separated species and hence that a virus would usually be pro- 

 duced only in related species. This is usually the case. 



The anatomical and physiological differences between species in- 

 creases as development proceeds, and embryos of different species are 

 much more alike than adults. It is quite possible that this similarity of 

 the embryos extends to the structure of the proteins and that embryo 

 proteins are more alike than adult proteins (cf. Pedersen, 1945). This 

 may account for the fact discovered by Murphy (1912) that heterol- 

 ogous transplants will grow in chick embryos. In general such trans- 

 plants do not grow when made in adults. 



The fact that chemical differentiation of the proteins proceeds more 

 or less parallel to the morphological differentiation of the organs is 

 clearly shown by the results of Burke et al. (1944). These workers 

 followed the development of organ antigens in growing embryos and 

 found that organ specificity was closely related to the growth of the 

 organs themselves. 



Woodruff and Goodpasture (1931) found that many viruses would 

 multiply in chick embryos, although they would not do so in the adult 

 chicken. These observations suggest that the proteinogens of embryos 

 are more closely related than are those of the adult animal and hence 

 are able to serve as precursors for heterologous proteins while the pro- 

 teinogens of the adult animal cannot. 



Moore, Shen, and Alexander (1945) have found that plasma from 

 chicken embryos contains principally one high molecular weight pro- 

 tein which is absent, or present in low concentration, in adult plasma. 

 Pedersen (1945) has found a characteristic protein (fetuin) to be 

 present in foetal calf plasma, and he has suggested that this protein 

 may be a precursor of the normal serum proteins. 



Burnet, Freeman, Jackson, and Lush (1941) and Dale have objected 

 to the assumption of a precursor on the grounds that the specific charac- 

 ter of the virus would then be determined by the host and not by the 

 virus, as is usually considered to be the case. 



The character of a virus which has been modified chemically is deter- 



