SECT, i] PHYSICO-CHEMICAL SYSTEM 303 



This is probably the most interesting consideration that emerges 

 from Table 27, but it may also be noted that the ash-content of the 

 white is just about half that of the yolk, a relation which would 

 practically be reduced to equality if the phosphorus in the yolk was 

 not taken into account. 



The presence of certain chemical elements of lesser biological im- 

 portance has been announced from time to time in a group of papers 

 which have some interest, although it is difficult to see, as yet, what 

 their importance is for the development of the embryo. Fluorine has 

 been estimated by Tammann and by Nickles, copper by Dhere, 

 boron by Bertrand & Agulhon, manganese by Bertrand & Medigre- 

 ceanu, iodine by Bonnanni and by von Fellenberg, lead by Bishop. 



These elements appear to be normal constituents of the egg. The 

 iron-content can be artificially increased by feeding iron-rich rations 

 to the hen, and iodine can also be introduced into the egg in this way, 

 as has been done by Bonnanni, Kreis and others, but Hofmann found 

 that though iron and iodine would enter the egg thus, it was impossible 

 to get copper to do so. In just the same way Ricci found it difficult 

 if not impossible to get As or Hg into the hen's egg by feeding sub- 

 toxic doses to the hen. The normal copper-content of the hen's egg 

 cannot be varied like its iron-content. The importance of iron in 

 the formation of haemoglobin is obvious, and the little that is 

 known about this process will be discussed in the section on pigments 

 in the embryo. Wassermann made a histochemical examination 

 of the egg-yolk and vitelline membrane for iron, and found a 

 relationship between the embryonic blood-islands and the iron of 

 the yolk. 



Some of the other data in Table 28 call for comment, Tammann's 

 1-13 mgm. per cent, fluorine in the fresh yolk works out at a quantity 

 of 0-2 mgm. per egg, and, as Zaleski found 0-23 per cent, fluorine 

 in the bones of the chick at hatching, o-o8 mgm. fluorine would be 

 required in the egg at the beginning, or less than half of what is 

 actually there. Zaleski's figure, however, is old, and may be too low. 

 It would appear, on the whole, as if the greater part of the fluorine, 

 iodine, copper, zinc, lead, aluminium, silicon and manganese is 

 localised in the yolk, and the greater part of the boron and arsenic in 

 the white. In view of the importance which we now attribute to these 

 less common elements as catalysts in living tissues, this distribution 

 may be found to have considerable significance. 



