H PHYSIOLOGY 



word, i.e. small sacs bounded by a wall of cellulose and containing 

 cell sap. Immediately inside the cellulose wall is a thin layer, the 

 primordial utricle, which encloses at one point a spherical or oval 

 structure known as the nucleus. If the section be taken from the 

 growing tip of a plant (Fig. 1), the cell sap is found to be wanting and 

 the cells consist only of the substance known as protoplasm, which 

 later on will form the primordial utricle. This with a nucleus is 



FIG. 1. General view of cells in the growing root-tip of the onion, from a longitudinal 



section, enlarged 800 diameters. ( WILSON.) 



a, non-dividing cells, with chromatin-network and deeply stained nucleoli ; 

 6, nuclei preparing for division (spireme-stage) ; c, dividing cells showing mitotic 

 figures ; e, pair of daughter-cells shortly after division. 



enclosed in a delicate cellulose wall. The wall is not an essential 

 constituent, since it is absent from many vegetable cells at some period 

 of their life and from animal cells generally. 



A better conception of the essentials of a cell can be obtained by 

 the study of a unicellular animal such as an amoeba (Fig. 2). This is 

 an organism frequenting stagnant pools, of varying size (from 0*1 to 

 0'3 mm. in diameter), apparently of a semi-fluid consistence. When 

 first examined it is generally spherical, but in a short time begins to 

 change its form, putting out processes known as pseudopodia. By 

 shifting the distribution of its material among these processes, it is able 

 to move about and also to ingest particles of food or pigment with 

 which it may come in contact. Near its centre a differentiated portion 

 can be distinguished which is known as the nucleus. The rest of the 



