CELLS 



51 



lemma, while fibrous sheaths bind the nerve fibres together. In Verte- 

 brate animals each nerve fibre usually has in addition a medullary 

 sheath. But even in the higher Vertebrates, " non-meduUated " or 

 simply contoured nerve fibres are found in the sympathetic and olfactory 

 nerves, and this simpler type alone occurs in hag, lamprey, and 

 lancelet, as well as in all the Invertebrates with distinct nerves. 



A nerve fibre contains numerous fibrils Uke those seen within a 

 ganglion cell. These are regarded by some as the essential elements in 

 conducting stimuli, while others maintain that the essential part is the 

 less compact, sometimes well-nigh fluid stuff between the fibrils, or that 

 the fibrils are but the walls of tubes within which the essentially nervous 

 stuff lies. 



The nerve fibres arise as prolongations of the ganglion cells, 

 which extend themselves in the embryo like Amoebae sending out 

 pseudopodia. 



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 small to be distinguished except 

 through lenses ; many Protozoa, e.g. large Amoebae, are 

 just visible to our unaided eyes ; the chalk-forming Fora- 

 minifera 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 

 distended with yolk. 



The typical and primitive form of cell is a sphere — a 

 shape naturally assumed by a complex coherent 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 cell, we may distinguish (see 



Fig- 25)— 



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

 sists partly of genuinely living stufi^ or protoplasm, and 

 partly of complex materials not really living (metaplasm) ; 



