6 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), etc. 



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- 



