SECT, i] PHYSICO-CHEMICAL SYSTEM 329 



90 per cent, to 10 per cent,, according to the species. Again, the 

 egg-membrane of the Brazihan gastropod studied by RofFo & Correa 

 is said, on the basis of qualitative tests only, to be a true keratin, 

 containing no reducing sugar and associated with no other sub- 

 stances, save 2*45 per cent, of ash. It contained calcium the amount 

 of which did not vary during development. 



The transparent horny egg-membrane of the selachian Mustelus ^ 

 laevis, which disappears half-way through the development of the ■ 

 embryo, has also been investigated by Krukenberg, who compared 

 it with the egg-membrane of the grass-snake, Tropidonotus natrix. The 

 former resembled the shell-membrane of the hen's egg rather than 

 the true keratin of the Myliobatis egg-case. The latter seemed to have 

 some of the properties of elastin and some of those of keratin ; from 

 it he was able to isolate a reducing carbohydrate as well as glycine, 

 tyrosine and leucine. 



Krukenberg was also one of the earliest workers to make quantita- 

 tive investigations on this subject. His figures for the protein of the 

 egg-shells of Murex trunculatus and the whelk Buccinum undatum, which 

 are given in Table 33, led him to make a new class of such substances, 

 the conchiolins. As no data exist for the sulphur content of most 

 of these proteins, it is impossible to say whether they are keratins 

 or not, and the whole subject needs re-investigation. About five years 

 later, Engel also investigated the egg-membrane protein of Murex, 

 and, obtaining 0-5 per cent, of sulphur from it, concluded, its other 

 properties taken into account, that it was a keratin. Engel also agreed 

 with Hilger, whose figures for the egg-membrane of the snake, 

 Coluber natrix (see Table 30), suggested an elastin as its principal 

 component. He had not been able to find any sulphur in it. About 

 the same time, Wetzel examined the conchiolin in_the_egg-shells of, 

 Mytilus edulis, and obtained from it, after hydrolysis, leucine, tyrosine, 

 glycine, Various hexone bases and ammonia, but no phenylalanine. 



The first efforts at quantitative discrimination between egg-mem- 

 brane proteins were contented with ascertaining the elementary 

 composition ; thus von Fiirth analysed the protein of Loligo vulgaris 

 eggs in this way (39 per cent, glucosamine), and Verson, and later 

 Tichomirov, decided that the egg-shell of the silkworm, Bombyx mori, 

 was a keratin-like body (3-7 per cent, of sulphur), though, owing to 

 its unusual properties, they called it chorionin. Of these two last- 

 named analyses, it is probable that Tichomirov's is the more accurate. 



