156 THE BIOSYNTHESIS OF PROTEINS 



embryonic development. Differentiation of cells of the neural crest into 

 pigment cells normally occur as a result of interactions with the mesoderm 

 which lies below. The appearance of the pigment cells can be prevented by 

 analogues of phenylalanine and, in explants of ventral mesoderm, different- 

 iation is obtained when phenylalanine is added to the medium. It seems 

 that the production of phenylalanine by the mesoderm and its utilization 

 by the ectoderm is an essential factor in the differentiation of these cells 

 (Wilde, 1955, 1956). Specific nutritional requirements have also been 

 observed in cultures of organs (Wolff, 1957). Differences in requirements 

 are simply a sign of chemical differentiation, but they also point to the 

 possibility that certain tissues might depend on their neighbours for simple 

 compounds. These elementary interactions may be of fundamental impor- 

 tance. Studies on the formation of specific proteins in such systems might 

 be illuminating. 



Pioneer work on the transfer of specificity by cell extracts and especially 

 by microsomes must be mentioned here. When liver microsomes are 

 spread on the chorioallantoic membrane of a developing chick embryo, a 

 thickening of the membrane is observed and the amount of glucose-6- 

 phosphatase, an enzyme characteristic of liver microsomes, increases 

 considerably (Le Clerc, 1954) as if the microsomes had carried over the 

 capacity of making this enzyme. More recently, Ebert (1959) implanted 

 on the chorioallantoic membrane a microsomal fraction from adult cardiac 

 muscle together with Rous sarcoma virus. In the tumour masses which 

 developed, muscle tissue was observed to form in 27 per cent of the 

 cases. These observations open new perspectives and a wide field of new 

 possibilities to the experimental study of differentiation. 



D. THE SYNTHESIS OF ANTIBODIES 



If bacteria, foreign cells, foreign proteins or polysaccharides are injected 

 to an adult rabbit, new y-globulins appear in the serum of the animal a 

 few days after the injection. These special y-globulins are recognizable by 

 their ability to form complexes with the injected substance or to bind 

 specifically on the surface of the injected foreign cells. These new y- 

 globulins are the antibodies, the injected cells or substances the antigens. 

 The antibodies are highly specific of the antigen which caused their appear- 

 ance in the blood. 



The antibodies do not arise by transformation of pre-existing y-globulins: 

 when i^C labelled normal y-globulins are injected into the blood of a rabbit 

 immunized against Pneiimococciis, the antibodies do not become labelled 

 (Gros et al., 1952; Green and Anker, 1954). On the other hand, when 

 labelled amino acids are injected to an animal which is producing anti- 



