Specific Mechanisms of Protein Synthesis in the Developing Chick Embryo 127 



to three dozen unincubatcd, embryonated White Rock eggs. The punctures 

 were sealed with paralTin wax and the eggs then incubated at 38° C under 

 conditions of controlled humidity. Starting with the fifth and ending with 

 the ninth day after the injection, embryos were harvested and a number pooled. 

 The mixture was homogenized for about three minutes in a Potter-Elvehjem 

 homogenizer in Ringer's isotonic saline solution, made up to 10 ml (fifth and 

 sixth days) or 20 ml (seventh through ninth days), and precipitated with tri- 

 chloracetic acid (final concentration, 8 per cent). Dry protein powders were 

 then prepared and counted (20). 



2. Is There Evidence for Selective Utilization of Egg-white or Yolk Proteins'} 



In the first set of experiments, chicken serum albumin injected into yolk 

 or egg-white was used as a protein tracer. Table I shows the results of two 



Table L Injection of Chicken Serum Albumin into Embryonated Eggs 



series of experiments. The spread of the data is indicative of the precision, 

 reliability, and reproducibility usually obtained in experiments of this sort. 



Let us now make the following assumptions: (a) that the injected protein is 

 a true tracer for egg-white and yolk protein respectively, i.e. that no permea- 

 bility or other pool barriers exist for its equilibration with the corresponding 

 unlabeled egg proteins; and (b) that there is no selectivity in the uptake mechan- 

 ism of the embryo either for or against a serum albumin tracer as a typical 

 precursor protein. Now we can calculate data shown in Table II and compare 

 the observed mean of the amount of protein actually formed, with that expected 

 on the basis of the above assumptions. The latter value is calculated by 

 multiplying the weight of total yolk or egg-white protein, about 3000 mg 

 each, by the per cent of the injected activity incorporated per embryo (from 

 Table I). 



There are profound discrepancies between the calculated and the observed 



