38 THE FOUNDATION Part I 



cord along the whole length of the leg. The largest single cell is the unfer- 

 tilized egg, commonly called the yolk, of an ostrich's egg. The egg cells of 

 birds, reptiles, and amphibians are all large because of the yolk stored in 

 them. Relatively large or small body cells are characteristic of different 

 groups of animals. Cold-blooded amphibians with low metabolism have 

 larger body cells than warm-blooded birds and mammals whose body tem- 

 perature and metabolism are high. A horse has smaller cells than a salamander 

 and literally lives faster because it has a relatively greater cell surface exposed 

 to body fluids bringing in oxygen and food and taking away waste. 



Differentiation of Cells. Diff'erentiation is a process of becoming different 

 and specialized. The skin of an embryo fish seems to be all alike; then scales 

 and glands develop in it. The possibility of difference was there, but it ap- 

 peared only under certain conditions. The epitheliomuscle cell of hydra has 

 become specialized for contractility at one end. Shapes and sizes of cells, 

 already mentioned, are results of differentiation. They are inherited patterns 

 brought out and also modified by the surroundings of successive generations 

 through the ages. 



Polarity of Cells. Polarity of a cell is consistent — difference between 

 opposite regions. It is a special kind of differentiation as in the epithelio- 

 muscle cell of hydra, one end useful as lining or as a gland, the other end 

 muscular. Polarity is almost universal in cells as it is in all living organisms. 

 Among the diverse examples are nerve cells in which the impulse enters 

 at one end and passes out the other, and gland cells in which the secretion 

 collects and passes out through the membrane at one pole. The polarity of 

 plants and animals is well known by the differences in the opposite ends as 

 in a turnip, a rose bush, or a donkey. 



Phases in the Life of the Cell 



Every cell goes through two phases: the first includes its growth, metabo- 

 lism, and characteristic activity, such as secretion; the second includes metab- 

 olism and reproduction by division. 



Interphase. The individual lifetime of the cell is known as the interphase. 

 It begins when the cell is produced by the division of a parent cell and lasts 

 until the cell itself divides or dies. The structure and general characteristics 

 of an animal cell have already been described and shown (Fig. 3.4). Further 

 mention of conditions in the nucleus should now be made. The nucleus con- 

 tains a tangle of threads of chromatin, the latter containing genes, the bear- 

 ers of hereditary traits. The chromatin threads are double, made up of two 

 slender strands, the chromonemata or colored threads in which lumps of 

 chromatin, the chromomeres, the probable locations of groups of genes, are 

 arranged irregularly. The two chromonemata are actually two future chromo- 

 somes lying so close together that the doubleness is difficult to discover. 



