130 



H. R. Mahler, H. Walter, A. Bulbenko and D. W. Allmann 



It can be seen that the relative specific activity of hearts is higher than that 

 of hvers when chicken heart homogenate is injected, whereas the relative 

 specific activity of the livers is higher than that of hearts when chicken-liver 

 homogenate is injected. 



Table V. Incorporation of Activity from Adult-Tissue Homogenates into Nine- 

 Day Embryos after Twenty-four-hour Incubation 



Counts per minute are within 5 per cent standard deviation. 



III. CONCLUSIONS 



The experiments on soluble protein tracers added to yolk and egg-white 

 demonstrate quite clearly that proteins added to the egg-white or, probably, 

 egg-white proteins themselves are incorporated with such low efficiency as 

 to rule out any important contribution from this source to the protein of the 

 developing embryo, at least up to and including the ninth day. Incorporation 

 of protein from the yolk is rapid, and soluble proteins injected into this source 

 may be utilized preferentially to some of the yolk proteins themselves. This 

 utilization of yolk rather than egg-white proteins as a source of embryonic 

 protein during this period is in accord with other investigations, notably the 

 quantitative protein depletion studies of Rupe and Farmer (21). For the 

 intervals studied, amino acids, peptides and proteins, even those of relatively 

 'foreign' origin such as the serum proteins, all apparently provide an equally 

 acceptable source of S^^ for embryonic protein synthesis (within an order of 

 magnitude or so), provided they are injected into the yolk. Now the protein 

 tracer must be diluted by at least a portion of the 3.0 g or so of yolk protein — 

 an estimate of approxim.ately 50 per cent would appear reasonable in view of 

 the results reported above. On the other hand, amino acids or peptides cannot 

 be diluted to any appreciable extent since the pools of these substances in the 

 egg are vanishingly small (22). From this one might conclude that proteins 

 themselves or substances easily formed from them must be the preferred precur- 

 sors of embryonic proteins. Since the egg protein ovalbumin is used no more 

 efficiently than the more "foreign" serum proteins, the pathways of assimilation 

 for these precursors, available to the embryo, must have at least some inter- 

 mediates in common. The data on peptides may find a similar interpretation. 



