Mendel Sc Leavenworth 

 O Hen 

 ® Duck 



1148 THE METABOLISM OF NUCLEIN AND [pt. m 



uracil (the so-called "yeast nucleic acid"), in the i8th-day chick 

 embryo, as well as the ordinary hexose nucleic acid similar to that 

 first isolated from the thymus. The chick embryo thus possesses the 

 power of synthesising both kinds of nucleic acid. The older obser- 

 vations of Mendel & Leavenworth therefore regain their importance, 

 for, using Tollens' method, they found an increase in the pentose 

 content of the incubated egg from zero to as much as 40 mgm, 

 (see Fig. 345). 



The first systematic investigation of the synthesis of nuclein bases by 

 the embryo was made by Fridericia, 

 who estimated them in the chick 

 embryo from the 8th day onwards, 

 using the Brugsch-Schittenhelm tech- 

 nique. He fully confirmed the earlier 

 statements that not more than traces 

 of purine bases were present in the 

 unincubated egg, e.g. 0-25 mgm. 

 purine nitrogen per egg. In order 

 to determine whether the presence 

 of any purine bases tightly enough 

 combined to resist acid hydrolysis had 

 been overlooked, Fridericia allowed 

 fresh eggs to autolyse, but did not 

 obtain in this way any more purine 

 nitrogen than by the previous 

 method. The small amount of nu- 

 cleoprotein in the fresh egg is doubt- 

 less to be accounted for by the blasto- 

 derm itself. His figures for increase of purine nitrogen in the embryo 

 are given in Fig. 346. Fridericia compared together in one and the 

 same graph the absolute amounts of dry weight, total nitrogen and 

 purine nitrogen. From the 8th to the i ith days the rate of increase 

 was much the same, but after that the dry weight and total nitrogen 

 curves drew away, and the purine nitrogen one was left slowly 

 increasing below them. This obviously indicated that the purine 

 nitrogen would decrease markedly in per cent, of the total nitrogen, 

 but Fridericia did not pursue the subject further. Its importance 

 will shortly appear. 



More recently, the question of nucleoprotein formation has been 



Days*5 



Fig. 345- 



