44 



THE ELEMENTS OF STRUCTURE. 



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; in multicellular organisms the 

 cells are combined into tissues and organs. 



Most cells are too 



Pi, 

 -CC, 



small to be distinguished 

 except through lenses ; 

 many Protozoa, e.g. large 

 Amoebae, are just visible 

 to our unaided eyes ; the 

 chalk - forming Foramin- 

 ifera 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. 17); 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 

 coherent substance situ- 

 ated 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 cell, we may distinguish (see 

 Fig. 21) 



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

 sists partly of genuinely living stuff or protoplasm, and 

 partly of complex materials not really living (metaplasm) ; 



FIG. 21. Diagram of ceH structure. 

 After Wilson. 



PL Plastids in cytoplasm. 

 cc. Centrosome. 



n. Nucleolus. 

 Ckr. Chromosomes. 



N. Nucleus. 



ct. General cytoplasm. 



V. Vacuole. 

 Gr, Granules. 



