8 GERM-CELL CYCLE IN ANIMALS 
are not supposed to form part of the living sub- 
stance; these are pigment granules, fat globules, 
excretory products, vacuoles (v), ete. 
It has been found possible to explain many cellular 
activities and even the results obtained by experi- 
mental animal breeding by studies of the physics 
and chemistry of protoplasm. An exhaustive ac- 
count of the subject is impossible and even unneces- 
sary here, but the importance assigned to the physico- 
chemical explanation of life phenomena requires a 
brief statement. Kossel has separated the cellular 
constituents into two main groups. (1) Primary 
constituents are those necessary for life; these are 
water, certain minerals, proteins, nucleoproteins, 
phosphatides (lecithin), cholesterin, and perhaps 
others. (2) Secondary constituents are not essen- 
tially necessary and do not occur in every cell; 
they are usually stored up reserve material or meta- 
bolic products representing principally what we have 
termed metaplasm. 
Water which constitutes about two-thirds of the 
animal is necessary for the solution of various bodies, 
the dissociation of chemical compounds, the exchange 
of materials, the removal of metabolic products, 
etc. Mineral substances are present in all animal 
tissues, and different tissues are characterized by 
the presence of different minerals. The principal 
ones are potassium, sodium, calcium, magnesium, 
iron, phosphoric acid, sulphuric acid, and chlorine. 
The other constituents are of a colloidal nature, 
and its richness in colloids is one of the chief charac- 
