4 THE BREEDING OF ANIMALS 



above, the cell in the animal body is in more or less close 

 physiological relation with the other cells of the body, 

 but these relationships and their bearings upon the 

 fundamental facts of biology have not yet been clearly 

 determined. 



5. The structure of the cell. The cell contains a 

 cell-body and the nucleus. The cell-body is all that 

 portion of the protoplasm not contained in the nucleus. 

 The cell in its simplest form is a rounded mass of proto- 

 plasm. This type is found generally in one-celled forms 

 and is the characteristic form of the egg-cell of the higher 

 animals. The fact that the form of cells in the higher 

 plants and animals is not always rounded spherical is 

 due to unequal pressure and the movement of the cells 

 comprising the body. 



The nucleus is a definite, clearly-marked body existing 

 within the protoplasmic contents of the living cell, and 

 its relation to growth, reproduction and heredity have 

 given it a commanding position in the study of modern 

 biological problems. Other bodies are often found in 

 the cell, such as food granules, products of excretion, 

 fat globules and crystals. None of these plays an active 

 part in the metabolism of the cell and may be regarded 

 as accidental or at least subsidiary to the major role 

 played by the protoplasm itself. Another body generally 

 found in the cell is the centrosome which is concerned 

 with the mechanism of cell division. The cell-wall is 

 generally present in the higher forms of plant and animal 

 life and consists of a membrane which is usually lifeless. 



6. Protoplasm. The protoplasm is universally present 

 in every living cell. It is the most fundamentally impor- 

 tant life substance. Huxley aptly designated protoplasm 

 as " the physical basis of life." It is not to be regarded 



