870 GENERAL METABOLISM [pt. iii 



In 1929 Friedheim studied the rH of chick embryo Breis from 

 different ages, using MichaeUs' mercury electrodes. There was no 

 paralleHsm between rH and growth-promoting power (measured by 

 the Carrel techniques), and the Breis seemed to be most reducing at 

 the 13th day of development; thus: 



The only investigation of the rH of avian yolk and white is that of 

 Pa\'lov & Issakova-Keo who used a platinum electrode. The yolk of 

 the infertile unincubated egg was always some hundred millivolts 

 more positive than the white: the average rH of the former was about 

 24, that of the latter 28. If the eggs were kept at room temperature 

 the EA of the white showed a continual tendency to become more 

 positive, reaching after a fortnight a level of + 0-42 or rH 32. If 

 incubated at 37° this positive trend occurred also in the absence of 

 development, but if the egg was fertile the white showed on the 

 contrary a negative trend leading to EA + o-io at 1 1 days, or rH 20. 

 The yolk followed a corresponding downward course, leading to 

 EA + 0-30 at 1 1 days or rH 18: thus always keeping at a less reducing 

 level than the white. These facts agree well with the well-known 

 negative trend found in cell-suspensions or bacterial cultures and 

 indicate that the developing embryo gives off reducing substances 

 to the rest of the egg. 



6-4. Water-metabolism of the Avian Egg 



It will be convenient to speak next of the water metabolism of 

 embryos, a subject of considerable importance. Although the bird's 

 egg forms one of the most complicated of the systems we have 

 to consider, it is yet one of the best understood, and for that reason 

 it may be taken first, the discussion then passing on to the eggs of 

 other organisms. Another reason for dealing with the hen's egg first 

 is that in it the embryo can be separated from the yolk at a com- 

 paratively early stage, so that separate determinations of water in 

 embryo and food-material can be done. This is much more difficult 

 in many other cases, such as that of the frog. 



In general we may say that from the earHest point examined the 

 chick embryo loses water relatively, although its actual content of 



