SECT, i] PHYSICO-CHEMICAL SYSTEM 327 



none in elasmobranch egg-cases. Krukenberg in 1885 decided that 

 the egg-case of Scyllium stellare was of a keratinoid nature, because 

 of its percentage composition, in which he found a marked amount 

 of sulphur. He observed the interesting fact that the egg-cases of 

 this fish, while still in the uterus of the parent animal, would dissolve 

 in pepsin and trypsin, while after they were laid they would not 

 dissolve in solutions of either enzyme. He also isolated tyrosine and 

 leucine firom the keratin of the egg-cases of Scyllium stellare. He made 

 very similar researches on the egg-cases of Scyllium canicula and 

 Myliobatis aquila, finding that they possessed rather different properties 

 and seemed to be of different constitution; thus on hydrolysis he 

 recovered a great deal of leucine and hardly any tyrosine from the 

 keratin of Scyllium canicula, while from the keratin of Myliobatis the 

 yields were precisely reversed. The latter substance was also con- 

 siderably more resistant to digestion than the former, and Krukenberg 

 considered that the former was not a keratin at all. He had already 

 decided (wrongly, as it turned out) that the shell-membrane of the 

 hen's egg was mucin, not keratin, and now he concluded that this 

 also applied to the egg-case oi Scyllium stellare, as well as to that ofLoligo 

 vulgaris, of which he made a separate examination. He thought it 

 possible also that the jelly which surrounds the egg in the ovo viviparous 

 selachians might be a mucin too, especially as, according to Schenk, it 

 was not precipitated by chromic acid, and he himself found that it was 

 extremely resistant to digestion by enzymes. This material has received 

 no further chemical investigation since the time of Krukenberg. 



Other workers who identified the proteins of egg-membranes 

 by the aid of colour tests and solubility reactions were Leuckart, 

 who showed, as far as anything could be shown with such preliminary 

 methods, that the membranes of planarian eggs were of chitin, and 

 Yoshida & Takano and Jammes & Martin, who drew a similar con- 

 clusion about the coats of the eggs of Ascaris lumbricoides, which they 

 found were readily soluble in gastric juice or in any acid.^ The 

 case of the parasitic nematodes is of special interest, for the chitinous 

 membrane does not arise until after the fertilisation of the egg, being, 

 therefore, in a sense, analogous to the fertilisation membranes of 

 echinoderms. Whether the chitin is formed as it is required during 

 these early stages, or whether it is already present in the unfertilised 

 egg-cell in some soluble form, is uncertain. Faure-Fremiet in an 



^ See also Campbell on the chitin of insect egg-membranes. 



