THE CELL 17 



The typical cell possesses a nucleus (Figs. 4, 5, 106). A few 

 rare and unusual exceptions exist. The nucleus is ordinarily a 

 spherical or ovoid body, centrally or peripherally located. It 

 is an organ of fundamental importance to the continued life of 

 the cell. If the nucleus is removed from the cell, as it can be 

 under favorable conditions, the cell may continue to live but not 

 for long, nor can it reproduce. It survives only as long as the 

 nourishment lasts which it has taken in before the operation; 

 this is about a week in the case of an amoeba. A typical cell 

 has one and only one nucleus, but there are numerous exceptions 

 of cells with several nuclei and some few types which have no 

 nucleus. The red corpuscle of man is a non-nucleated cell. 

 When the corpuscle starts life in the marrow of the bone it has 

 a nucleus; but as it grows and migrates out into the blood stream 

 the nucleus disappears. Whether or not the little sac of hemo- 

 globin that remains is to be regarded as a living cell is as yet an 

 unanswered question. The sieve tube in the wood of plants 

 transports food; it is a living cell with active protoplasm, but 

 when mature it has no nucleus. Certain cells possess two or 

 more nuclei. The cells of the mushroom are binucleate, and 

 the polymorphonuclear white blood cell has typically several 

 nuclei. There are unusual cases of cells with as many as six or 

 more nuclei (Fig. 10a). 



The typical nucleus contains a tiny body called the nucleolus, 

 or "little nucleus." There may be several nucleoh (Figs. 5, 10a, 

 106). The functions of the nucleolus are unknown, though 

 many have been attributed to it. 



Another important cell structure, much more conspicuous and 

 probably more common in plants than in animals, is the vacuole, 

 poorly named, for a vacuum is impossible in the living world. 

 The name roughly carries the idea of a cavity. The vacuole in 

 most plant cells is a single, large, centrally located sac filled with 

 salts, sugars, and organic substances in aqueous solutions (Fig. 4). 

 It plays a very important part in the life of the cell. Owing to 

 its significance, its widespread occurrence, and variations in 

 form, we shall return to it for a more detailed discussion later. 



Among the conspicuous plant-cell inclusions are the chloro- 

 plasts ("green bodies") which occur in all green tissues of plants, 

 therefore most commonly in leaves (Fig. 4). These little globules 

 contain chlowphyll, the substance primarily responsible for the 



