322 



EFFECT OF CHEMICAL AGENTS 



[Cn. XI 



nuclei are subjected to hydrochloric acid and potassium ferricyanide they 

 immediately assume a deep azure-blue color, clearly due to the formation 

 of "Prussian blue " or Fe 4 (Fe C 6 N 6 ) 3 . This series of reactions can only be 

 explained on the ground that there is iron in the nucleus. 



Iron has shown itself to be essential to the growth of all 

 organisms. Its importance for growth is indicated by its rela- 

 tive abundance in the yolk of birds' eggs, and by the fact of 

 its occurrence in larger proportion in a mammal just born than 



No Fe 



FIG. 89. Echinus larvae from parallel cultures, all 5* days old. a, reared in a solu- 

 tion containing all salts, including iron as FeCl 3 ; b and c, from solutions contain- 

 ing all salts excepting iron. (From HERBST, '97.) 



in later stages. When excluded from a nutritive solution 

 which is otherwise complete, growth is imperfect (Fig. 89). 

 This may be associated with the facts that in plants the chloro- 

 phyll granules are not developed in its absence, that in the 

 higher animals haemoglobin cannot be formed, and that the 

 chromatic substance of all cells requires it. 



The question in what form iron is absorbed by the organism 

 has been the subject of an extensive discussion. Although 

 doses of metallic iron have long been used with favorable 



