34 THE FOUNDATION Part I 



multicellular animal begins its existence as a single cell which soon divides 

 into two. Each of these grows and divides into two, and thus in the majority 

 of the cells the repeated growing and dividing go on as long as the animal 

 increases in size, whether it is a flea or a cow. This reproduction of cells is 

 entirely independent of sex. 



The characteristics of a many-celled animal are the expressions of its cells 

 acting together. A bird flies and its sensory cells react to light, gravity, and 

 air currents; its nerve cells carry messages to and from the brain; its muscle 

 cells contract; its body consumes more oxygen and releases more energy as 

 flight demands it. The responses of its cells are the links between the bird 

 and the world about it. 



Structures and Functions. Interphase means that the cell is in a phase of 

 life between divisions. In this phase, also called the resting stage, the cell is 

 resting from division. It is not in any sense resting from respiration and 

 other routine metabolic processes. Certain structures are typical of animal 

 cells though all are not necessarily present in every kind (Fig. 3.4). Some 

 plant cells do not have an organized nucleus and the chromatin is naked in 

 the cytoplasm. 



Nucleus. The nucleus is essential to the growth and reproduction of the 

 cell. It is usually clearly defined and sharply bounded by a thin, scarcely 

 visible membrane. It contains a foundation of nuclear sap in which definite 

 structures are suspended. In living cells the nuclear sap looks watery; in 

 prepared cells it often shrinks away leaving open spaces. With rare exceptions, 

 the nucleus alone contains chromatin, the physical basis of heredity and the 

 most remarkable substance of protoplasm. The delicate, darkly staining 

 threads, the chromonemata or color threads form a webby network in the 

 nuclear sap. They represent the future chromosomes. One or more minute 

 spherical bodies, the nucleoli, are often conspicuous during the interphase; 

 their substance disappears during cell division, much of it being incorporated 

 in one or more chromosomes. 



The importance of the nuclei has been shown by removing them from 

 living cells and noting the results. An ameba can be cut in two so that only 

 one part contains a nucleus. After such treatment the part without the nucleus 

 will live for some days, will respire, digest its food, and move about but it 

 doe» not grow or reproduce. On the other hand the part containing the 

 nucleus grows, replaces the lost part, and finally divides as usual. All well- 

 established cells have nuclei at some time during their life history. The red 

 blood cells of man and other mammals have no nuclei when mature as they 

 usually are when in circulating blood. However, nuclei are always present 

 when the cells are first formed. 



Cytoplasm. As already defined, the cytoplasm is all of the cell except the 

 nucleus. The ground substance of cytoplasm is a clear semifluid, the hyalo- 



