14 



PHYSIOLOGY 



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 forui the primordial utricle. This with a nucleus is 

 enclosed in a delicate cellulose wall. The wall is not an <ssrnt ial 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 



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 ; 

 &, nuclei preparing for division (spireme-stage) ; r, dividing cells showing mitotic 

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



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

 frequenting stagnant pools, of varying size (fromO'l to 0'3 mm. in diamctrr). 

 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 

 amoeba, the protoplasm or cytoplasm, often presents further differentiation 

 into an outer clear layer and an inner finely granular substance. The latter 

 may contain coarser granules, some of food material, others apparently 

 formed in situ by the surrounding protoplasm, and often small vacuoles 

 (' contractile vacuoles ') which are continually altering their size and serve 

 to keep up a circulation of fluid in the interstices of the cytoplasm. In all 

 cells, whether animal or vegetable, with whieh we are acquainted, this 



