B.—CHEMISTRY 45 
polysaccharide containing three different kinds of hexose residues. The 
constitution and chain length have been investigated and the following 
model is presented for this interesting type : 
| : a 
QJucose wnir Galactose writs 98 Idese or Altrose 
wenir, 
These examples serve only to illustrate the manifold character of the 
polysaccharides which are capable of existence and suggest that we are 
at the beginning of our knowledge not only of the structure of this 
group, but also of the functions which such polysaccharides serve in 
Nature. 
Nothing could be more important than the development of the more 
recent discoveries of polysaccharides possessing immunological functions. 
Several of these have been recognised as antigens, and a knowledge of 
their composition, properties, and intimate structure must obviously be 
of immense service. The properties of the specific polysaccharide of 
type III pneumococcus may be imitated by a degraded fragment of arabic 
acid derived from gum arabic (gum acacia). In our general investigation 
of gums, experiments have proceeded a considerable distance in the direc- 
tion of unravelling the complex structure of arabic acid. This is con- 
posed of arabinose, rhamnose, galactose and glucuronic acid residues, 
and the mode of assembly of these different sugars will, it is expected, soon 
become clear. The glucuronic acid residues are intimately associated 
with a galactose chain and the union occurs through position 6 of the 
latter sugar. The pentose units appear not to be vitally concerned with 
this nuclear portion of the molecule and are capable of removal by scission 
without disturbance of this essential nucleus. 
The carbohydrates of immunological interest are not simple in structure 
and probably have a molecular weight much above 2,000. The most 
characteristic, though by no means the only repeating unit, is represented 
by a bionic acid which in the case of the degraded arabic acid contains 
a glucuronic acid residue linked through the sixth position of galactose, 
a mode of union which resembles that which occurs in gentiobiose : 
