504 THE CELL AND MAMMALIAN HISTOLOGY 



Paramecium, of molluscs, of the frog and of man, and sperm tails 

 of many phyla, all have the same structure. Within a sheath 

 there are two central strands, and a ring of nine surrounding 

 them, making eleven in all. Each outer strand is sometimes split. 



TISSUES 



The resemblance of some parts of the body to a woven cloth 

 led M. F. X. Bichart, at the beginning of the nineteenth century, 

 to apply to them the French word tissu, of which the English 

 form is tissue ; the name stuck, and a tissue may be defined, 

 to continue the weaving metaphor, as any part of the body 

 with a recognisable texture. The study of the tissues is called 

 histology, from a Greek word meaning cloth. It has been chiefly 

 developed for mammals, because of its medical importance, and 

 the rest of this chapter is concerned primarily with them. Much of 

 the descriptive matter, however, applies to other vertebrates, 

 and the general principles probably apply to all Metazoa. 



CELL TYPES 



The traditional classification of tissues is based on their appear- 

 ance, but in recent years a more logical division, founded on 

 the type of cell of which they are chiefly composed, has become 

 possible. In the second decade of this century it was discovered 

 that most cells and tissues could be kept alive, apart from the body 

 which formed them, provided that they were given a supply of 

 oxygen and organic food, which is usually derived from blood 

 serum. Such cells are said to be growing in vitro, i.e. in glass vessels, 

 instead of in vivo, in the living body, and the technique of dealing 

 with them is called tissue culture. Some of the original cultures, 

 made from chicks, are still alive, having outlived many genera- 

 tions of their species. Under some conditions of growth tissues 

 in vitro maintain their original form, but under others they 

 undergo a process of dedifferentiation, in which the cells continue 

 to grow but do not form organised structures. To some extent 

 they change their form, and cells from different parts of the body 

 come to resemble each other more closely, but it is this which 

 helps us in classification. All cells in tissue culture belong to 

 one or other of three main types, called the epitheliocyte, the 

 mechanocyte and the amoebocyte (Fig. 393). These are not only 

 distinct morphologically, but also behave differently, and it i§ 



