THE METABOLISM OF EGGS, I 65 



than I, 0-69-0-89, in the unfertilized eggs of Urechis caupo (Horo- 

 witz, 1940); but Brachet (1934a) obtained the value 0-99 for the 

 unfertilized frog's egg. These results certainly do not suggest a 

 common endogenous substrate in unfertilized eggs of different 

 species. 



Intuitively, one feels that profound changes in metabolism 

 should occur at fertilization and that these may be reflected in the 

 R.Q. of the egg. In the period 5-20 minutes after fertilization of 

 the sea-urchin egg, the R.Q. is o-66, but in the first 2-10 minutes, 

 even lower values are obtained. The average R.Q. for the first 

 30 minutes after fertilization is 0-84. These results were obtained 

 by Laser & Rothschild (1939) using the Warburg indirect method. 

 Low values for sea-urchin eggs at somewhat longer times after 

 fertilization have been obtained by Ephrussi (1933), Borei (1933) 

 and Ohman (1940), while Brachet (1950) has reported that the 

 R.Q. of the frog egg falls from 0-99 to 0-66 after fertilization; these 

 latter measurements were done over a very long period, 10-15 

 hours, and are not, therefore, strictly relevant to the issue of what 

 metabolic changes occur at fertilization. Cleland (1950a) found 

 no change in the low R.Q. of rock-oyster eggs at fertilization, but 

 Horowitz (1940) states that the low R.Q. which is characteristic 

 of unfertilized Urechis eggs disappears at fertilization, and that 

 during the first two hours of development, the R.Q, is i. These 

 results are probably not so confusing as they superficially seem to 

 be, when one remembers the diff'erent times after fertilization that 

 measurements have been made and the difference in the mor- 

 phology of a fertilized egg, 10, 30 and 120 minutes after fertiliza- 

 tion. In this book we are only concerned with the early phases of 

 reproduction, the first 60 or so minutes of the egg's existence 

 following fertilization. Metabolism after the fusion of the pro- 

 nuclei and energy sources during development (Needham, 1942; 

 Brachet, 1950) are not, therefore, considered. 



Acid production. The sudden evolution of acid, about 20 /x- 

 moles/ioo mg. N, when the sea-urchin egg is fertilized, was first 

 mentioned in 1929 by Ashbel and examined in detail by Runn- 

 strom (1933). Reference to Fig. 13 shows that the evolution of this 

 acid, or at any rate the bulk of it, is of very short duration, about 

 five minutes. The nature of the acid is unknown; it is neither 

 lactic, pyruvic nor malic acid and, according to Yeas (1950), it is 

 unlikely to be any of the Krebs cycle acids. In spite of Cennamo 



