18 I. LESLIE 



DNA-P, as Bieth et al.^ noted in the case of chick embryo brain. Higher 

 values for the PNA-P/DNA-P ratio in the adult as compared with the em- 

 bryonic tissues have been reported for the cerebral cortex of the guinea pig,^^ 

 skeletal muscle of the chick, ^"^ and rat liver.^^ The ratio in fetal rat liver,^^ 

 although not in embryonic chick liver,^^ corresponds to those for adult 

 reticulo-endothelial tissues (Tables II and III). The PNA-P/DNA-P ratio 

 is constant in rat kidney, over a period in which cell number increases by 

 60 %}^^ Brodyi^^ reports that the ratio is highest during the logarithmic 

 growth phase of the human placenta. 



In Drosophila larvae, the salivary glands are composed of about 150 cells 

 which can undergo appreciable enlargement without dividing. In their in- 

 teresting study of this tissue, Patterson and Dackermann''^ report an in- 

 crease in the average PNA and PN content per cell, but not in DNA content 

 during doubling of cell size (Table IV, ref. 42). A similar situation occurred 

 in the development of spermatocytes in the insect, Arvelius alhopunctatus,^^^ 

 and in the doubling of nuclear volume without increase of DNA content 

 during estrogen stimulation of rat uterus. ^"^ 



The PNA/DNA ratio decreases during the early development of Arhacia 

 eggs, because at this stage the total amount of PNA per egg remains un- 

 changed, while the DNA content steadily increases.^^ Using a microbiolog- 

 ical method ^^ which measures the total deoxyriboside content of amphibian 

 eggs, Hoff-j0rgensen and Zeuthen"" found that this does not change during 

 the initial segmentation division of the fertilized ovum. These apparently 

 contradictory observations could be reconciled if a conversion of an initial 

 store of free deoxyribosides to polymerized DNA occurred in the first stages 

 of embryonic development. 



Although these and other results^"' ^^^ appear to rule out the possibility 

 of the conversion of PNA to DNA, as originally proposed by Brachet,"' 

 Agrell has recently reported an inverse relationship between the PNA and 

 DNA contents of the pupae of Calliphora erythorocephala during meta- 

 morphosis."* As the DNA decreases, the PNA increases during histolysis 

 of larval tissues ; by the completion of histogenesis, the DNA content has 

 increased again, and the PNA content diminished. 



06 D. S. Robinson, Biochem. J. 52, 628 (1953). 



"« N. B. Kurnick, J. Exptl. Med. 94, 373 (1951). 



" S. Brody, Exptl. Cell Research 3, 702 (1952). 



o« F. Schrader and C. Leuchtenberger, Exptl. Cell Research 1, 421 (1950). 



09 M. Alfert and H. A. Bern, Proc. Natl. Acad. Set. 37, 202 (1951). 



10 E. Hoff-J0rgensen and E. Zeuthen, Nature 169, 245 (1952). 



" C. A. Villee, M. Lowens, M. Gordon, E. Leonard, and A. Rich, /. Cellular Comp . 



Physiol. 33, 93 (1949). 

 '2 R. Abrams, Exptl. Cell Research 2, 235 (1951). 

 13 J. Bracket, Compt. rend. soc. biol. 108, 813, 1167 (1931). 

 1* I. Agrell, Nature 170, 543 (1952). 



