II GAMETOGENESIS AND FERTILIZATION yig 



clearly, at least for phosphorus compounds, that the principal site of synthesis 

 of yolk material is the liver of the hen; such materials are then transported by the 

 blood and selectively absorbed by the ovarian egg. An active role of the follicle 

 cells and perhaps also of the vitelline membrane in the process is implicit in the 

 failure of the uterine egg to incorporate 32P into organic compounds. Telfer (1954) 

 has demonstrated also that at least antigen 7 of the Cecropia silkworm is secreted 

 into the blood by another tissue and is incorporated into the yolk by the ovary. 



Kavanau (1954a), however, deduces from observations on amino acids of sea 

 urchin eggs allowed to stand in sea water, that "the proteins of the egg are probably 

 synthesized by the egg itself, the ovary merely supplying the free amino acids and 

 small peptides". It is not possible to generalize along these lines for other iso- 

 lecithal, nor indeed, for other invertebrate eggs. 



The relationship between phosphatide deposition and protein deposition may 

 be of functional significance; Smith (1952) feels that phosphatide fat of the trout 

 may be important in transmission of protein from the yolk to the embryonic cells 

 and "may be correlated with consumption of protein as an energy source". 



The demonstration of adult proteins in the egg has created questions of two 

 sorts : (j) the method of utilization of the yolk proteins in embryo formation and 

 (2) the extent to which adult proteins are represented in the yolk. Mystkowski 

 (1936) examined the early chick embryo for cathepsin activity and found that 

 the yolk sac activity was 15 to 20 times as great as that in the embryo proper. 

 He was unable to demonstrate synthetic activity in vitro. He concludes with 

 justifiable reserve that "the problem of utilization of egg proteins by the embryo 

 is not yet satisfactorily solved". Goldstein and Guintsbourg (1937) separated the 

 embryo, yolk and each of the extraembryonic membranes and assayed extracts 

 of each for proteolytic activity. With gelatin as a substrate, proteolytic activity is 

 inexplicably low during the first days of incubation. It appears at 6-8 days with 

 a maximum at 13-14 days. 



Recently, Brierly and Hemmings (1956) demonstrated that hens actively 

 immunized to Salmonella pullorum systematically incorporate antibodies in the yolk, 

 so that the yolk titre is rather constantly in the ratio of i : 8 to that of the adult 

 serum. Not only is there a device for incorporating the antibodies into the yolk, 

 but they are also selectively transported across the yolk sac membrane. Injection 

 of fowl serum immune to S. pullorum or Brucella abortis gives rise to circulating 

 titres in the embryo at hatching. Moreover, the yolk sac is selective, for immune 

 rabbit, cow and horse sera do not give rise to circulating titres, and serum of the 

 pigeon, though transmissible, has a lower titre. However, even homologous adult 

 serum antibodies suffer extreme loss in the transmission, and the fate of the 

 disappearing antibodies has not been ascertained. 



Since the eutherian egg is essentially yolk-free, the question of incorporation 

 of adult proteins into the egg is at once more difficult and less urgent. The work 

 of Brambell and coworkers on transfer of large molecules to the embryo is, 

 therefore, the more interesting. Rogers and Hemmings (1949) find that "the 

 maternal plasma proteins pass in considerable quantities into the yolk sac cavitv 

 through the bilaminar omphalopleur of rabbit embryos of the 7th and 8th days 

 post-coitum, that they retain their identity while doing so, and that there is 



Literature p. 744 



