44 



THE ELEMENTS OF STRUCTURE. 



longalions of the ganglion cells ; according to others, the neuroglia or 

 oilier ensheathing elements contribute to the extension of the nerve 

 fibres, or rather special neuroblast cells make both sheath and fibre. 



IV. Cells. In discussing tissues, it was necessary to 

 refer to the component cells. Let us now consider the 

 chief characteristics of these elements. 



A cell is a unit mass or area of living matter usually with 

 a nucleus. Most of the simplest animals and plants 

 (Protozoa and Protophyta) are single cells ; eggs and male 



elements are single cells 

 p? in multicellular organisms 



the cells are combined 

 into tissues and organs. 



Most cells are too small 

 to be distinguished except 

 through lenses; many Pro- 

 tozoa, e.g. large Amoebae, 

 are just visible to our 

 unaided eyes ; the chalk- 

 forming Foraminifera are 

 single cells, whose shells 

 are often as large as pin- 

 heads, and some of the 

 extinct kinds were as big 

 as half-crowns (see Fig. 

 1 7) ; the bast cells of 

 plants may extend for 

 several inches ; the largest 

 animal cells are eggs dis- 

 tended with yolk. 



The typical and primi- 

 tive form of cell is a 

 sphere, a shape naturally assumed by a complex coher- 

 ent substance situated in a medium different from itself. 

 Most egg-cells and many Protozoa retain this primitive 

 form, but the internal and external conditions of life 

 (such as nutrition and pressure) often evolve other shapes, 

 oval, rectangular, flattened, thread-like, stellate, and so on. 

 As to the structure of a eel/, we may distinguish (see 

 Fig. 21)- 



(a) The general cell substance or cytoplasm, which con- 



FlG. 21. Diagram of cell structure. 

 After Wilson. 



PL Plastids in cytoplasm. 

 cc. Centrosome. 

 n. Nucleolus. 

 CJir. Chromosomes. 

 N. Nucleus. 

 ct. General cytoplasm. 

 V. Vacuole. 

 Gr. Granules. 



