326 THE UNFERTILISED EGG AS A [pt. m 



membranes of snake's eggs which show all variations as to lime-content 

 (see Table 9) are, as Kelly has shown, composed of amorphous and 

 unstable calcium carbonate. The eggs of gastropods, such as Helix, 

 Ampullaria, Bulimus, Amphidromus, etc., are, as Turpin {Helix aspersa) 

 and Rose {Helix pomatia) , besides the workers mentioned above, have 

 demonstrated, like those of birds in having their lime in the form 

 of calcite. For a general theory explaining these differences see the 

 paper of Prenant. 



The shells of eggs may also contain calcium phosphate. In the 

 hen and in birds generally there is very little, but the globules seen 

 in their egg-shells are believed to be calcium phosphate, though no 

 analysis has given a figure of more than i per cent, of this salt. 

 In other eggs, however, there may be more; thus Gmelin found 

 7-3 per cent, in the egg-shells of a tortoise, and Kelly noted its 

 presence also in those of Bulimus and Lophohelia, though she gives no 

 analytical figures. 



It is interesting to note that the mineralogical form of lime in the 

 egg-shell may vary during the development of the embryo; thus 

 Kelly says that the shell of many full-grown mollusca is conchite, 

 while that of their respective embryos and eggs is calcite. Kelly found 

 that the organic substance was a remarkably constant proportion 

 of the shells of mollusca, reptilia and birds (see Table 9). Some egg- 

 coverings contain almost no water at all (birds), others have more 

 than the egg-contents, as has been shown for the trout's egg by 

 Kronfeld & Scheminzki (membrane 75 per cent., egg 66 per cent.). 



By far the commonest substance of which egg-membranes are com- 

 posed is keratin, though this protein seems to take many forms, and 

 not to have exactly the same properties in different situations. The 

 earlier workers were content to assert the presence of it on the basis 

 merely of solubility tests. Thus in 1874 Schenk studied the egg-shell of 

 Raia quadrimaculata, and decided that it was 95 per cent, keratin after 

 the application to it of the protein colour reactions and an examina- 

 tion of its behaviour towards various solvents. The same conclusion 

 was arrived at by the same methods by Hussakov & Welker for the 

 egg-cases of Raia erinacea, and the Port Jackson shark, Heterodontus 

 philippi. The keratin of these egg-cases was insoluble in all solvents 

 except acid and alkali. They found that sulphur was present, but 

 no phosphorus, and they were unable to find any reducing sugar 

 after total hydrolysis. Irvine, using an optical test for chitin, found 



