RECENT ADVANCES IN SCIENCE 29 



bolism each of these acids can be changed into the other. The 

 removal of tryptophan or of vitamines from the diet produces 

 an even greater nutritional disturbance than the absence of 

 the two above-mentioned substances, but there is no corre- 

 sponding falling off in the excretion of allantoin, which strongly 

 supports the view that arginine and histidine play an impor- 

 tant part in purine metabolism. Work on somewhat similar 

 lines is being carried out by Osborne and Mendel (/. Biol. 

 Client., 191 7, 29, 69) who have shown that corn gluten is 

 deficient in lysine and tryptophan and is therefore not capable 

 of promoting the growth of white mice unless supplemented by 

 the addition of other proteins containing these acids as, for 

 example, caseinogen, lactalbumen, edestin, beef, brewers' 

 grains, etc. The best protein for the purpose is lactalbumen. 

 It suffices to add relatively small quantities of these supple- 

 mentary proteins, and in any case much less than would be 

 sufficient to maintain growth in the animals experimented on. 



As a further illustration of the significance of the various 

 constituents of ordinary food, a paper by W. Stepp (Zeitsch. 

 Biol. 19 1 6, 66, 365) may be quoted. It is there shown that 

 lipoids such as lecithin, kephalin, cerebrone, cholesterol, etc., 

 are essential to life, and that dog biscuits from which lipoids 

 and vitamines have been removed by extraction with alcohol 

 are unable to maintain white mice in health, and that neither 

 vitamines nor lipoids alone are sufficient to render such biscuits 

 suitable for the maintenance of life. 



Further light has recently been thrown on the constitution 

 of yeast nucleic acid. The nucleic acids in combination with 

 protein form the highly important nucleoproteins, the chief 

 constituents of cell nuclei. The two best-known nucleic acids 

 are those derived from the thymus gland and from yeast and 

 the prevailing opinion is that all nucleic acids are identical 

 with one or other of these two compounds. By hydrolysis 

 thymus nucleic acid can be split up into phosphoric acid, 

 two purine bases guanine and adenine, and two pyrimidine 

 derivatives thymine and cytosine, together with levulinic acid 

 derived most probably from a hexose ; the products of hydro- 

 lysis of yeast nuclei are essentially the same, only that the 

 places of thymine and levulinic acid are taken by uracil and 

 ribose respectively. Partial hydrolysis of thymus nucleic acid 

 yielded a so-called nucleotide C n H 17 N2POi which is a com- 



