98 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



appropriate for his purpose, rearranges them in such a way that the 

 new house has its own special architectural features, and discards as 

 waste the bricks and stones which are not suitable. This idea under- 

 lies the term so often used by German writers, who speak of the 

 cleavage products of protein as Bausteine (building stones). Each 

 tissue has special architectural features in its protein molecules, and 

 these molecules are reconstructed by using the Bausteine that pre- 

 viously had been used in the building of other protein molecules, 

 either in another animal or in vegetable structures. The Bausteine 

 which are in excess or are unsuitable are simply got rid of as waste 

 substance. 



A large number of the Bausteine are never actually built into 

 protoplasm, but are converted by the liver into urea, and this is dis- 

 charged from the body via the urine (see Urea formation, p. 148). 



FIG. 26. Section of the villas of a rat killed during fat-absorption (E. A. Schiifer) : 

 ep, epithelium ; str, striated border ; c, lymph cells ; c', lymph cells in the epithelium ; 

 Z, central lacteal containing disintegrating lymph cells. 



One can only conjecture at present which are the ones that on 

 p. 52 we compared to diamonds, because they are unusually precious 

 for the synthesis of protein by tissue cells ; but probably phenyl- 

 alanine and its near relation tyrosine come into this category, for if 

 they are injected into the blood-stream they do not give rise to any 

 increase in the urea formed. 



