PROTOPLASM 13 



substances to form compounds. The best-known example is 

 Mucin (see p. 26). 



(6) Ferro-proteids. In the pigment of the blood (Haemo- 

 globin) proteids occur linked to an iron-containing molecule 

 (p. 199). 



(7) In horn (Keratin) a sulphur- containing molecule joined 

 to the proteid gives the special characters to the substance 

 (p. 23). 



