SECT, i] PHYSICO-CHEMICAL SYSTEM 321 



It would be as well to emphasise the fact that no principle of selec- 

 tion has been used in the preparation of Table 30, on the ground 

 that results such as those of Roffo & Correa on a Brazilian gastropod, 

 and McCrudden on fresh-water fishes, which seem obviously wrong, 

 may not be so at all. The estimation methods and analytical processes 

 which are by general consent judged most satisfactory at the present 

 time cannot be considered in any way final, and to have excluded 

 certain results on account of the technique employed in obtaining 

 them would not have been justifiable. Table 30 does not, therefore, 

 absolve investigators fi'om the duty of looking up the original papers 

 in such cases as touch them most closely, and forming an independent 

 judgment, according to the best opinion of the time, on the stress 

 which can be laid upon them. It is needless to say that I leave out 

 of account all doubtful figures in the generalisations made here. 



I -12. Egg-shells and Egg-membranes 



Very little is known about the relative proportions of yolk, white, 

 and shell, in the eggs of the lower animals, or rather, in most cases, 

 egg-contents and shell or surrounding membrane. Table 32 gives a 

 few figures. The discrepancy between the results of Ford & Thorpe, 

 on the one hand, and Wetzel, on the other, is very strange, especially 

 as they both used Scyllium canicula eggs, but it is probably due to 

 insufficiency of the statistical element. Ford & Thorpe's proportions 

 are more likely to be accurate. 



Much work, however, has been done on the membranes and hard 

 coverings which invest the unincubated eggs of diflferent kinds of 

 animals. For instance, the gelatinous substance which surrounds the 

 undeveloped amphibian egg was examined chemically by Brande 

 in 1 810, who noticed that it absorbed water and was not precipitated 

 by tannin or by strong acids. Later work has shown that it consists 

 almost entirely of mucoprotein and water. Wetzel's figures for its 

 weight are shown in Table 32. Giacosa isolated mucin in a pure 

 state from it in 1882, and the figures which he obtained for its per- 

 centage composition are shown in Table 33. He was able to show 

 the presence of a reducing sugar on hydrolysis, but he could 

 isolate nothing else from the jelly, and therefore concluded that 

 it was pure mucin. The presence of glucosamine in the muco- 

 protein was afterwards confirmed by Hammarsten, by Schulz & 

 Ditthorn and by Wolfenden, who confirmed Giacosa's finding that 



