STRUCTURE OF CELLS. 55 



need not be further dwelt upon at present, but one of them 

 should be briefly considered. It has been stated that drops of 

 liquid often appear in the protoplasm e.g., sap, oil, etc. These 

 drops are sometimes single, in which case the drop often becomes 

 so large as to occupy the entire central part of the cell, the proto- 

 plasm becoming reduced to a layer (sometimes extremely thin) 

 which lines the cell-membrane (cf. Nitella^ p. 28), and is known 

 as the primordial utricle (see p. 28). In other cases a number 

 of drops appear in the same cell. If these increase greatly in 

 size, as they often do, they may occupy the greater part of the 

 cell, the protoplasm being reduced to a more or less fine network 

 which connects with a peripheral layer (cf. Tradescantia, p. 30). 

 The cavities in which the drops lie are called vacuoles, and the 

 cells are said to be vacuolated. 



Nucleus. The form of the nucleus is tolerably constant, 

 being usually round or oval ; but sometimes it may be irregular 

 or elongated. It may contain one or more nucleoli, and the 

 character of the internal network (p. 50) varies extremely. 

 During the division of cells (as in the developing egg of the star- 

 fish) the nucleus undergoes marvellous transformations (known 

 as karyokinesis or mitosis) which are still imperfectly understood, 

 and are the object of active research at the present time. 



Cell-membrane. The variations of the membrane demand a 

 somewhat fuller notice at this point. Wholly 

 wanting in many young cells, it first appears 

 as a delicate film outside the cell-substance, 

 and may continue to increase until its thick- 

 ness is enormous (Fig. -i). Chemically con- 

 sidered, it differs in plants and animals. In 



, 



plants it is usually composed of a carbohy- 

 drate substance known as cellulose (C 6 H 10 O 5 ), FlG . 25 . (After Sachs) 

 which is closely related to starch. In ani- Poiien-grain (a single 



. isolated cell) of Cichari- 



malS it IS Composed OI nitrOgenOUS SUb- m, showing external 



stances of unknown composition. As the S^S^ f the ceU " 

 cell grows older the membrane may undergo 

 many changes, both in configuration and in chemical compo- 

 sition. Various mineral deposits may take place in its sub- 

 stance, or on the surface ; or its substance may undergo a direct 

 transformation into wood (I iyn (fica tion), horn, gelatine, mucus, 

 and any one of a great variety of other substances. For instance, 



