RESULTS OF ANALYSIS 79 



and commenced to spin on August 6th. In other terms, a silk-worm 

 during the period of its life as caterpillar, consumes 12 '6 grams of 

 fresh mulberry leaf or 4-0 grams of dry leaf, of which I '3 grams is 

 assimilated, in a period of twenty-five days. The amino acids in 

 mulberry leaves have been estimated by Katayama [1916]. There is 

 a marked difference in the proportions of the different units (see p. 123). 

 Of the three chief constituents of silk, alanine is present in large amount 

 in the mulberry leaf, but there is very little glycine ; tyrosine is evi- 

 dently present only in small amounts as no yield is mentioned. These 

 data suggest that glycine may be synthesised de novo and that tyrosine 

 arises from phenylalanine. 



Gelatin, Spongin, Elastin. 



Gelatin contains no tryptophan, cystine, or tyrosine, but it contains 

 more glycine than any other protein, except elastin. It also con- 

 tains a large amount of proline and oxyproline. Levene and Beatty's 

 analysis [1906] was not made by the ester method. This protein 

 appears to have no similarity to silk-gelatin, which contains so much 

 serine. 



Spongin resembles gelatin in its high content of glycine, but differs 

 in its content of glutamic acid. 



Elastin seems to be made up almost entirely of glycine and leucine ; 

 like gelatin, it contains no tyrosine, but it differs from gelatin in con- 

 taining more phenylalanine. No di-amino acids could be isolated by 

 Bergh [1898] and Hedin [1898], but a minute quantity of arginine 

 was obtained by Kossel and Kutscher [1898]. 



Keratins. 



The keratins, except for certain of the proteins usually in- 

 cluded in this sub-group, are remarkable for containing more cystine 

 than any of the other proteins ; in human hair cystine exists to the 

 extent of about 14 per cent., in other keratins its amount varies from 

 2-8 per cent. Tyrosine is also present in fair quantities, and* the 

 amounts of leucine and of glutamic acid are high. 



The amount of cystine isolated from the keratins is generally far 

 below the actual quantity present. The cystine content has been 

 ascertained by the determination of the total sulphur content of the 

 hydrolysed protein and of the loosely-bound sulphur (see Part II.) 

 [Morner, 1901-2], or by the determination of the sulphur content of 



