1. Post-deposition growth of the black snake embryo is characterized by k values as follows: days 1-11, 0.46; days 11-34, 0.057; days 34-67, 0.039.

2. Total excreted nitrogen is 12.55 mg. occurring successively in development as ammonia, urea and uric acid; k value of total nitrogen production is 0.062, days 11-67, which in comparison with those of growth during this period suggest that stored protein is an energy source during a large part of development.

3. Urea is excreted into the albumen which is the principal storage reservoir and into the yolk; that which is excreted into the yolk is reabsorbed after the 45th day and re-deposited in the albumen as urea, and is in part (22%) converted to uric acid.

4. Concentration of urea in the yolk and yolk-sac continues to increase at the same rate after the 11th day (k = 0.024), although the actual amount declines after the 45th day; concentration in the embryonic tissues increases to the 58th day, then decreases sharply to hatching time; concentration in the albumen increases throughout development, reaching a concentration of approximately 500 mg.%.

5. Uric acid synthesis is believed to be preceded by urea formation, and the presence of urease in the foetal liver and kidney suggest that the urea is hydrolysed to ammonia which is then incorporated into uric acid. Site of the transformation is uncertain, though the early and persistent localization of uric acid in the chorio-allantoic membrane points to this as the organ of synthesis. Decrease in total urea content is quantitatively identical with increase in uric acid.

6. The significance of these findings in relation to development of the vertebrate cleidoic egg is discussed.

The author is grateful to Prof. C. A. Kind for many valuable suggestions, and to Mr Robert E. Beyer for making the uric acid assays reported herein.