298 ADVENTURES IN RADIOISOTOPE RESEARCH 



not identical with those derived from the yolk, but are synthesized 

 in the embryo. 



The formation of labelled phosphatides in growing eggs was investigated 

 by Hevesy and Hahn [1938]. It was found that the phosphatides 

 present in the yolk are taken up from the plasma by the ovary and 

 incorporated into the latter; as soon as the yolk leaves the ovary no 

 more change occurs in the content or composition of its phosphatides. 

 When labelled phosphate is administered to a hen after the yolk has 

 left the ovary and is located in the oviduct, the egg takes up active 

 phosphate but no active phosphatide is formed. In experiments in vitro 

 as well, eggs placed in radioactive sodium phosphate solution take up 

 active phosphate Vmt no active phosphalides are formed. The slight 

 activity of the phosphatides present in the yolk of incubated eggs is 

 presumably due to the influx into the yolk of small amounts of active 

 phosphatides synthesized in the embryo. This view is supported by the 

 fact that the ratio of the specific activities of the embryo phosphatide 

 P and yolk phosphatide-P was much larger (3000) in the 6 days experi- 

 ment than in the 16 clays experiment (100). The activity of the residual 

 P of the yolk, which is mainly composed of vitellin and nucleoprotein, 

 was larger than that of the phosphatides; this can be understood if we 

 admit the possibility that the extraction of the strongly active, non- 

 protein constituent of the yolk is not quantitative, for in this case the 

 specific activity of the residual P would be increased. 



The embryonic residue obtained after extraction of the acid-soluble 

 and ether-soluble constituents is composed chiefly of nucleoproteins. 

 That the specific activity of the nucleoprotein-P is the same as that 

 of the inorganic P extracted from the embryo is not surprising, because 

 much less nucleoprotein is present in the yolk than in the embryo (Table 

 1). The greater part of the nucleoproteins present in the embryo must 

 therefore have been built up in the course of incubation; during this 

 process labelled phosphate has an opportunity of entering the nucleo- 

 protein molecules. 



Distribution of radioactive phosphate in the egg 



The greater part of the sodium phosphate injected into the white 

 is still found at the end of the 6 days experiment in that part of the 

 egg. The distribution of the activity between white, yolk, connecting 

 fluids (which were not, however, free from wliite and yolk) and embryo 

 is seen in Table 6. 



The low activity of the yolk might possibly be due to a slow rate of 

 penetration of the vitellin membrane by the phosphate ions; this point is 

 under investigation. Another possible explanation is that the inorganic 

 P content of the yolk is lower than that of the white. If a distribution 



