132 VETERINARY BACTERIOLOGY 



been offered by physiologists for the phenomenon of cell nutrition. 

 Food substances carried to the cell by the blood must pass through 

 the vessel and cell-walls and be anchored there if they are to be 

 used in any of the processes of cell metabolism. According to 

 Ehrlich, the protoplasm must be made up of molecules having an 

 affinity for a great variety of food materials. These molecules 

 he conceives to be made up of a central portion surrounded by 

 atomic groups which unite with certain food molecules and bind 

 them to the cell. These atomic groups have affinity for certain 

 food substances, and, therefore, are differently constituted. These 

 atomic groups he calls side-chains or, better, cell receptors. The 

 character of these receptors may be illustrated by a chemical 

 analogy. Benzene has the following formula: 



H H 



H C C H 



\ / 



c=c 

 / \ 



H H 



Any one of these hydrogens may be substituted by some element 

 or group. Suppose one such to be replaced by the carboxyl 

 group (COOH), another by the amino group (NH 2 ), another by 

 the aldehyd group (CHO), and still another by a hydroxyl 

 group (OH). Such a hypothetical compound might be illustrated 



as follows: 



H COOH 



W 



s % 



OH C C NH 2 



/ \ 



H CHO 



There are several possible ways in which such a compound 

 might react. Should an alkali be brought into contact with it, 

 the base would be taken up by the carboxyl atom group. Acids 

 would be bound by the amino group. Other substances wouM be 

 bound by other atom groups. The cell receptors must be con- 



