498 MOLECULAR MECHANISMS OF DIFFERENTIATION 5 



used as a precursor for protein formation in the host organ either directly, or after 

 intermediary degradation to amino acids. Complete absence of activity in other 

 host tissues would have indicated that no amino acids were liberated from the 

 proteins of the graft and that these proteins or their polypeptide degradation prod- 

 ucts were taken up specifically by the homologous host organ. The fact that 

 small amounts of activity did appear in non-homologous host tissues implies that 

 either some donor protein was incorporated as such even in non-homologous 

 tissues or that amino acids liberated in the breakdown of the graft proteins enter 

 the circulation and are incorporated non-specifically into cellular proteins of het- 

 erologous host organs Recent experiments by Walter, AUman and Mahler (1956) 

 seem to corroborate Ebert's findings. The relationship between this specific uptake 

 of homologous proteins and the stimulation of mitosis by homologous tissue sus- 

 pensions (Andres, 1955) remains, as yet, to be explored. 



It is noteworthy to add that substantial amounts of peptides can be identified 

 in protein free extracts of sea urchin and amphibian embryos. Published photo- 

 graphs of chroma tograms seem to show in the case oi Hyla regilla an increase in the 

 intensity of these spots from stages 2-7 to stage 18, while in Rana pipiens the inten- 

 sity of the peptide spots decreases during the same progression of developmental 

 stages (Berg, 1950; Eakin, Berg and Kutsky, 1950; and Kutsky, Eakin, Berg and 

 Kavanau, 1953)- Recently, large amounts of peptides were identified in extracts 

 of Triton alpestris (Chen, ig56a). Peptides with growth promoting activity were also 

 described by Gustafson and Hjelte (1951) and Gustafson, Hjelte and Hasselberg 

 (1952), Kavanau, et al. (1954) and Hjelte, et al. (1955) in extracts of sea urchin 

 embryos. Since in fully differentiated cells peptides seem to be present only in 

 much smaller quantities than are found in embryonic tissues, the question arises 

 as to whether peptides play a more significant role in protein metabolism of at 

 least some embryonic tissues. 



{b) Amino acids as precursors 



Amino acids are probably used as precursors of proteins throughout embryo- 

 genesis. To what extent utilization of amino acids or of larger protein building 

 blocks supplement each other and under what condition utilization of the one 

 or the other precursor predominates is not known. The presence of free amino 

 acids in extracts of sea urchin and amphibian embryos was detected chromato- 

 graphically by Berg (1950); Eakin, Berg and Kutsky (1950); Kutsky, Eakin, Berg 

 and Kavanau (1953) and Chen (1956a), but little progress has been made in 

 correlating the observed changes in the amino acid content of these extracts with 

 protein formation. On the other hand the question of the free amino acid content 

 of sea urchin embryos was investigated by Gustafson and Hjelte (1951), who 

 found rather striking changes occurring during the mesenchyme blastula and 

 gastrula stage; they suggested that these changes are correlated to the apparently 

 increased synthesis of proteins as manifested by the appearance of new enzymes 

 (Gustafson and Hasselberg, 1951 and Gustafson, 1954) and new antigenic proteins 

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



A systematic investigation of the amino acid content and its possible correlation 

 to protein formation in the developing sea urchin embryo was attempted by 



