Specific Mechanisms of Protein Synthesis in the Developing Chick Embryo 129 



Table IV. Incorporation into Embryos of Proteins and 

 Peptides Injected into the Yolk* 



Expressed as per cent of injected activity recovered per embryo. 



4. Is There Evidence for Organ-specific Transfer? 



In order to test the hypothesis of organ-specific transfer advanced by 

 Ebert we have attempted to extend investigations of this sort to the use of 

 S^^-labeled aduh chicken Hver and heart homogenates. These were prepared 

 from deep-frozen organs of a White Leghorn hen injected with a mixture of 

 S^^-amino acids, and treated as described above. 



After several months the tissues were thawed and homogenized in a tris- 

 (hydroxymethyl)-aminomethane buflfer solution at pH 7.4 containing 0.9 per 

 cent KCl, first in a Waring blender and then in a Potter-Elvehjem homogenizer. 

 The liver and heart homogenates, made up to 10 per cent (weight/volume) 

 with the same buffer solution, were then treated with cysteine at a pH of 8.0 

 to 8.5 to assure removal of all exchangeable S^^. After dialysis, some undis- 

 solved material was removed by low-speed centrifugation, and the relatively 

 clear supernatant fluid was used for intravenous injection into 9-day-old 

 chick embryos. Embryonated White Rock eggs were incubated at 38° C 

 under controlled humidity conditions for a period of 9 days. They were then 

 candled, and the location of the blood vessels was marked on the shell of each 

 egg. An area of about 1 cm^ of the shell above the vessel was carefully cut out 

 by means of a dental drill and burr without injuring the membrane, and the 

 small square was removed with a razor blade. A drop of mineral oil was placed 

 on the membrane to render it transparent, and 0.1 ml of the liver or heart 

 homogenate was intravenously injected in the direction of blood flow. The 

 eggs were reincubated for 24 hours and the embryos were excised. Hearts 

 and livers were removed, the organs were pooled, and homogenized; dry 

 protein powders were prepared for counting as described before. Similarly 

 aliquots of the homogenates used for injection were prepared and counted. 



The results of these experiments are given in Table V. In all, two series 

 of experiments make up the Table. In the first series, twenty-four embryos 

 each were injected with heart and liver homogenates; of these, twenty-two and 

 eleven respectively survived. 



In the second series, forty-four out of forty-seven embryos injected with the 

 heart preparation survived, while the number of survivors was twenty-two 

 out of twenty-eight for the liver homogenate. Thus the table summarizes 

 data obtained on 99 survivors out of 123 embryos that were injected: 66/71 

 for heart; 33/52 for liver. 



