40 SECTIONAL ADDRESSES. 
nucleotides. ‘The latter differ from one another in respect of their 
nitrogenous factors, which are guanine (2-amino-6-oxypurine), adenine 
(6-aminopurine), uracil (2 : 6-dioxypyrimidine), and cytosine (2-oxy-6- 
aminopyrimidine), giving their names to the four nucleotides linked with 
each other in the following manner : 
(HO),P0.0.C;H;02.C;H,ON; (guanine) 
ui 
(HO poo MHS Oae ON, (cytosine) 
(H0):P0.0.0,41,0.0,1.N, (adenine) 
i 
(HO),P0.0.6;8;0,.0.1,0.2%, (uracil) 
We owe this picture of plant nucleic acid to the combined researches 
of many chemists, conspicuous amongst whom is A. Kossel; he derived 
purine bases from nucleins in the early eighteen-eighties, and subse- 
quently identified the products of completely hydrolysing the nucleic 
acids from yeast and from the thymus gland. Characterisation of 
intermediate products in such hydrolyses—namely, the nucleotides of 
guanine, cytosine, adenine, and uracil—with the corresponding nucleo- 
sides, guanosine, cytidine, adenosine, and uridine is due chiefly to 
W. Jones and to P. A. Levene, with whom was later associated W. A. 
Jacobs; but the most picturesque of all contributions to the subject 
was made by the earliest of the systematic investigators, Friedrich 
Miescher, who followed his isolation of nuclein (nucleic acid) from pus 
cells (1868) by the remarkable discovery that the spermatozoa heads of 
Rhine salmon consist almost entirely of protamine nucleate (1874), and 
that this must have arisen, not directly from food, but from muscle 
protein. 
Whilst the yeast cell and the wheat embryo have the power to 
synthesise nucleic acid of the structure represented above, the thymus 
gland elaborates another nucleic acid in which a hexose is substituted 
for d-ribose, and uracil is replaced by thymine, its methyl derivative 
(5-methyl-2: 6-dioxypyrimidine); the order and mode of nucleotide 
linkage are also different. These nucleic acids, although deriving their 
carbohydrate and phosphoric acid from the nourishment on which the 
organism thrives, do not owe the purine factors to the same source; 
in other words, the tissues must have power to synthesise a purine 
ring. The mechanism by which they exercise this power is one of the 
many problems which await elucidation, but arginine (a-amino-é- 
guanidinevaleric acid) has been indicated as one possible origin, whilst 
histidine (a-amino-f-imidazolylpropionic acid) may be a source of the 
pyrimidine nucleus. 
The transformations undergone by nucleic acid in contact with 
tissue-extracts have provided the subjects of numerous investigations 
extending over thirty years. In fact, the experimental material is of 
