THE NUCLEIC ACID CONTENT OF TISSUES AND CELLS 17 



The concentration of PNA-P per unit PN is one feature which shows the 

 same downward trend for all the embryonic tissues (Table V). In these 

 tissues (brain, heart, liver, and muscle) the protein is accumulating more 

 rapidly than the PNA, in contrast to the more even balance that is main- 

 tained during the postnatal growth of rat liver. However, when the body 

 weight of the rat is between 20 and 50 g., the rate of PNA accumulation ex- 

 ceeds that of protein, resulting in a peak in the PNA-P/PN ratio''^ 



In chick embryo tissue, the relative accumulation rates of various P con- 

 stituents were obtained by plotting the total amounts against total DNA-P 

 and so obtaining the value k in the heterauxetic equation y = hx''}^ In 

 liver and brain, the rates for PNA-P and PN slow down to new constant 

 values between the fourteenth and fifteenth days, the accumulation of PNA 

 lagging behind that of protein in each phase. For the first five days of devel- 

 opment of the chick embryo, Herrmann^^ reports a close correspondence 

 between the accumulation rates of PNA, DNA, and PN. It is in the later 

 stages, during the differentiation of the embryonic tissues, that the protein 

 is building up more rapidly than the PNA. During the early development 

 of amphibian embryos Brachet^"" reported simultaneously high concentra- 

 tions of PNA, DNA, and ATP in the dorsal half of the gastrula when this 

 was developing more rapidly than the ventral half. 



From decreasing concentrations of DNA-P per unit weight and per unit 

 PN, one can deduce that cell size and protein content are steadily increas- 

 ing. This is the case in chick embryonic brain, liver, and muscle, but not 

 in heart tissue, which is morphologically fully developed at the eighth day 

 of incubation. 101 In the cerebral cortex of the guinea pig, the DNA concen- 

 tration per unit weight of cortex is seven times more at the twenty-fifth day 

 of gestation than in the adult," a fact which agrees with the observed 

 changes in number of cell nuclei per unit volume of cortex. ^^^ 



On the assumption that the increase in DNA content of brain tissue is 

 actually measuring increasing cell number, Mandel and Bieth^"^ reported 

 that the cell number of rat brain reaches its final adult level 16 days after 

 birth. Guinea pig brain has developed to this stage at the time of birth, cat 

 and rabbit brain at 1 month, dog brain at 5 months, and human brain at 

 approximately 1 year.^"* 



In embryonic chick brain and adult rat liver, there is a three-and five- 

 fold increase in the PNA-P per cell over their respective developmental 

 stages (Table V). Such changes are, of course, reflected in the ratio PNA-P/ 



•00 J. Brachet, Cold Spring Harbor Symposia Quant. Biol. 12, 18 (1947). 



•01 F. R. Lillie, "The Development of the Chick." Henry Holt and Co., New York, 



1908. 

 '02 V. B. Peters and L. B. Flexner, Am. J. Anal. 86, 133 (1950). 

 '03 p. Mandel and R. Bieth, Experientia 7, 343 (1951). 

 104 P. Mandel and R. Bieth, Compt. rend. 236, 485 (1952). 



