76 FERTILIZATION 



ammonia, Orstrom observed that the NHg-binding capacity of 

 fertiHzed sea-urchin eggs was higher than that of unfertiHzed eggs. 

 This observation has been confirmed by Hultin (1950c) who says 

 that the maximum rate of ammonia uptake occurs during the 

 formation of the fertiUzation membrane. It may seem curious 

 that, in the absence of added ammonia, this is precisely the time 

 at which the eggs are producing the maximum amount of ammonia. 

 When sea-urchin eggs are allowed to develop in the presence of 

 ^^N-glycine or ^^N-DL-alanine, and subsequently extracted with 

 KCl or sucrose solutions, insoluble proteins, which are probably 

 derived from the microsomes, are found to incorporate the isotope 

 rapidly in the early phases of development, while the soluble 

 proteins do not (Hultin, 1953a). Hultin interprets these results as 

 showing that soon after fertilization, there is a 'rebuilding or multi- 

 plication of small, cytoplasmic granules, containing ribonucleic 

 acid' (p. 18), a process which is intensified during determination. 

 In spite of the advances made in recent years in accumulating in- 

 formation about nucleotide, protein, and amino acid metabolism in 

 developing sea-urchin eggs, it is unfortunate that Orstrom's 

 original work has not been systematically repeated and, therefore, 

 remains largely unconfirmed. Even the transient production of 

 ammonia at fertilization has not been observed by other workers 

 and if this experiment could be done fifteen years ago, it could 

 easily be repeated now. 



Kavanau (1953, 1954^, b) has made interesting observations 

 on the amino acid metabolism of sea-urchin eggs. He observed 

 cyclical changes in amino acid concentration (free, peptide bound, 

 and derived from proteins), in fertilized eggs of Paracentrotus 

 lividus, but not in the eggs of Strongylocentrotus purpuratus ; the 

 failure to detect these changes in the latter may have been due 

 to unavoidably poorer experimental conditions. The experi- 

 ments were done rather a long time after fertilization, from 

 the point of view of this book, though the fall in protein in- 

 soluble in a mixture of o-iM-KCl and o-oiiM-acetic acid, in total 

 protein, and in total amino acid concentration, two to three hours 

 after fertilization, is striking. Kavanau believes that an abrupt 

 change in amino acid metabolism occurs at fertilization, yolk 

 synthesis stopping and intense yolk-protein degradation starting, 

 the latter being a major source of energy. In the case of unfer- 

 tilized eggs Kavanau observed an increase in protein and a decrease 



