SECT, i] PHYSICO-CHEMICAL SYSTEM 331 



simply an insoluble modification of ichthulin. As he pointed out, 

 industrial use has long been made of insoluble forms of proteins, 

 such as casein, and there was no reason why the egg-membranes 

 of certain eggs, at any rate, should not be insoluble modifications 

 of the proteins of their yolks. Steudel & Osato also suggested that 

 the ovomucoid of the egg-white of the hen might be a phylogenetic 

 reminiscence of the mucoprotein with which the amphibian egg is 

 surrounded. For a review of this work see Steudel. 



The eggs of salps and tunicates are surrounded by a coat of very 

 much smaller cells which act as some sort of protection for the 

 developing embryo inside. Zavattari has demonstrated histochemi- 

 cally the presence of an abundance of glycogen in these test cells, 

 and believes that they have a nutritive function. If so, this would 

 be a third case where such an active participation of the shell or 

 case in embryonic metabolism would have been noted, the two others 

 being the abstraction of calcium from the shell of the hen's egg, and 

 the contribution of amino-acids by the egg-case of the silkworm. 



A good deal is known about the osmotic and other properties of 

 the membranes of amphibian and fish eggs, but these are so intimately 

 associated with the physico-chemical processes taking place during 

 development that consideration of them will be postponed to Sec- 

 tion 5. It will suffice to mention here the experiments of Peyrega, 

 who found that the egg-cases of Scyllium canicula were permeable 

 to salt. He fitted up osmometers with small pieces of the case as 

 the membranes, and observed that it took about 20 days to establish 

 osmotic equilibrium with respect to solutions of sodium chloride 

 about as strong as sea water, when distilled water was put on the 

 other side. These egg-cases have also been shown by Needham & 

 Needham to be permeable to urea and ammonia. 



1-13. Proteins and other Nitrogenous Compounds 



The principal protein substance which is found to occur in the 

 eggs of all known animals closely resembles the vitellin of the 

 hen's egg. It has even been found, according to Chatton, Parat & 

 Lvov, in the food-reserves of infusoria. The early analyses of the eggs 

 of the pike by Vauquelin in 181 7, of the barbel {Cyprinus barbus) by 

 Dulong d'Astafort in 1827, and of the trout [Salmo fario and Cyprinus 

 carpio) by Morin in 1823, ^^^ to no more than the view that an 

 albuminous substance w£is present in them. But with the work of 



