THE CHEMICAL NATURE OF DIFFERENTIALS 575 



discussions. We have already referred to the polysaccharides which are 

 found in cell constituents in many kinds of bacteria and which are character- 

 istic of certain types and species of bacteria, especially of pneumococci. They 

 were first discovered in pneumococci by Heidelberger. Each type of pneumo- 

 coccus has its own kind of polysaccharide. These complex carbohydrates 

 function, as a rule, as haptens, which in combination with foreign proteins 

 may act as full antigens ; but the polysaccharides of Types II and III pneumo- 

 cocci may, as such, act as antigens. As Heidelberger has shown, the polysaccha- 

 ride of Type III pneumococcus consists of numerous units of cellobiuronic 

 acid, while a single unit is antigenically ineffective, a combination of several 

 units may unite as antigen with the specific antibody in anti-pneumococcus 

 type III horseserum. It is possible that within the bacterial cells these 

 polysaccharides are combined with proteins. In the case of the pneumo- 

 cocci it can be shown that they are not diffusely distributed within the cell, 

 but form a constituent of the bacterial capsule. As far as a comparison is 

 possible between simple unicellular and higher, very complex organisms, 

 these carbohydrates may be compared to organ differentials of higher 

 organisms rather than to organismal differentials; they are localized in 

 certain parts of the cells and as a rule act as antigens only in combination 

 with other substances ; but at the same time they are specific for group and 

 also for species of these unicellular organisms in the same way as organ 

 differentials may carry a species differential. It appears probable that also in 

 bacteria protein substances situated within the cell body are the carriers of 

 the typical species and class differentials, and quite recently Heidelberger 

 and Kendall have begun to separate such substances by methods which pre- 

 vent or diminish much their hydrolysis during the process of preparation; 

 some of them may be fully antigenic. 



Within cells there arise also the enzymes, endoenzymes and exoenzymes, 

 as they might be called, which likewise show various kinds of specificities. 

 From a functional point of view, their most marked specificity relates to the 

 substratum on which they act and which they convert into different substances, 

 either by splitting or by synthesizing processes. Enzymes are characterized by 

 this specific effect, by the conditions under which they act, and by their place 

 of origin. In accordance with their intimate connection with cells, they consist 

 of proteins which in some instances may function in combination with 

 prosthetic groups, especially also with certain vitamins. These proteins have 

 been obtained in crystalline form (Sumner, Northrop, Kunitz and others). 

 It has been shown that some enzymes develop from precursor substances 

 which also have been obtained in crystalline form (Northrop, Kunitz) ; thus 

 pepsin, trypsin and chymotrypsin are derived from pepsinogen, trypsinogen 

 and chymotrypsinogen. In addition, there has been distinguished among the 

 pancreatic proteolytic enzymes, heterotrypsin and beta and gamma chymo- 

 trypsin. The substratum specificity of these enzymes goes farther than has 

 been assumed, and Bergmann has shown that simple peptides can be found 

 on which the various proteolytic enzymes of the pancreas exert a specific 

 splitting effect. It is the enzyme itself which may convert the precursor sub- 



