10 GERM-CELL CYCLE IN ANIMALS 
ciates into electro-positive Na ions (cations) and 
electro-negative Cl ions (anions). Colloidal par- 
ticles are likewise electrically charged, those of acid 
colloids usually negatively and those of alkaline 
colloids positively. The union and separation of 
particles and their consequent rearrangement cause 
gelation, liquefaction, etc.; it is thus evident that 
many physiological activities may be due to the 
electrical charges of ions instead of the chemical 
nature of the particles themselves. Cellular struc- 
tures therefore depend upon the tendency of col- 
loidal particles to form aggregates (gelation, coagula- 
tion), and more or less upon the electrically charged 
nature of the particles. 
The most characteristic chemical constituents of 
protoplasm are the proteins. The most common 
proteins in the body show on the average the follow- 
ing percentage of elements : — 
Carbon ok +s ik . 50 -55 % 
Hydrogen . . . . . - 6.5- 7.3% 
Nitrogen. . . . . whtyy - 15 -17.6% 
OxySeny io . 19 -24 % 
Sulphur. . 2... 2. 3 3- 24% 
Proteins may be separated into three groups: (1) 
simple proteins, such as protamines, albumins, and 
globulins; (2) conjugated proteins, the glucopro- 
teins, nucleoproteins, and chromoproteins; and 
(3) the products of protein hydrolysis, infraproteins, 
proteoses, peptones, and polypeptides. These have 
been studied both by microchemical and macro- 
chemical methods. In the former method reagents 
are applied to the microscopic objects and the 
