INTERCELLULAR SUBSTANCES AND WALLS 77 



many protoplasts. This essentially static viewpoint has been sup- 

 planted by a broader picture of extracellular and intracellular 

 substances as participants in and reflections of cellular proces- 

 ses. We may treat the cell wall as a member of a great class 

 of substances which are formed by a physiologically unique (and 

 important) part of the protoplast— its surface. Nor need we re- 

 quire for our purposes that a cell elaborate particular products 

 homogeneously distributed at all of its surfaces. Certainly this 

 situation does not fit the epidermal cells of the green plant which 

 deposit specialized waxes and other cuticular materials on one 

 face only. 



If the functions of cells are to be understood, full account must 

 be taken of the many extracellular and intercellular substances 

 produced by organisms, with the restriction, for present purposes, 

 that these products retain a spatial relationship to the protoplast 

 surface from which they originated. 



In the search for broad principles upon which to base an under- 

 standing of the cell wall, we must examine the array of substances 

 that various organisms produce at their surfaces without taxonomic 

 restriction. 



In pursuit of our objectives, we will have the opportunity to 

 examine a sample ranging from the enterobacteria to Mammalian 

 bone. 



Bacteria and Fungi 



The bacterial protoplast is characteristically enclosed within 

 a rigid wall of complex constitution. 



The isolation of bacterial wall substance has only recently been 

 accomplished by combined procedures including mechanical dis- 

 ruption, washing, centrifugation, and enzyme treatment (trypsin, 

 ribonuclease, pepsin). The purity of wall isolates may be established 

 by electron microscopy, which reveals cytoplasm-free preparations 

 resembling collapsed balloons. 



Analysis shows that such preparations commonly contain lipids, 

 ester phosphate, hexoses, hexosamines, and amino acids. 



More specifically, major cell wall constituents include the fol- 

 lowing: 



