38 THE ELEMENTS OF STRUCTURE. 



IV. Cells. 



In discussing tissues, it was necessary to refer to the 

 component cells. Now we shall consider the chief charac- 

 teristics of these elements. 



A cell is a unit mass of living matter. Most of the 

 simplest animals and plants (Protozoa and Protophyta) are 

 single cells ; eggs and male elements are single cells ; but in 

 the multicellular organisms the components are closely com- 

 bined into tissues and organs. 



Most cells are too small to be distinguished except 

 through lenses ; Amxba, Paramecium, and many Protozoa 

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

 minifera are single cells, whose shells are sometimes as large 

 as pin-heads, and some of the extinct kinds were as big as 

 half-crowns ; the bast-cells of plants may extend for several 

 inches ; the largest animal cells are eggs distended with yolk. 



History. — The word " cell " was first iised in histological description 

 by Hooke (1665), and Grew (1671-5), but not in a very accurate or 

 definite way. Malpighi (1675) also described minute "utricles," some of 

 which we should call cells. 



Leeuwenhoek (Phil. Trans. 1674) seems to have been the first to 

 describe single-celled organisms. In the eighteenth century the analysis 

 continued ; thus Rosel von Rosenhof described the ' ' Proteus animalcule " 

 or Ammba in 1755, and Fontana, in 1784, discovered the kernel or 

 nucleus of the cell. 



But the fact that Bichat, in his Anatomic Generate (1801), speaks of 

 tissues only, shows that the import of cells was not realised at the 

 beginning of this century. 



In 1835, Robert Brown showed that a nucleus was normally present 

 in all vegetable cells, and in the same year Johannes Miiller definitely 

 compared the cells of plants with those of the notochord in animals. 



The cellular structure and origin of organisms began to be vaguely 

 recognised by many. At length, in 1838-9, Schwann and Schleiden 

 showed that all but the simplest plants and animals are built up of cells, 

 and develop from cells, thus establishing the famous " cell theory" : — 

 " There is one universal principle of development for the elementary part 

 of organisms however different, and this principle is the formation of 

 cells."* 



* Those interested in history should read the scholarly history of cell-lore by Sir 

 William Turner, " The Ceil Theory, Past and Present," Inaug. Address to Scottish 

 Microscopical Society (Edin. 1890, also in Nature, 1890). See also Professor 

 M'Kendrick On the Modem Cell Theory (Proc. Phil. Soc, Glasgow, 1888) also his 

 text-hook of Physiology. The articles Mori-hologv and Protoplasm in the 

 Encyc. Brit., and the article Cell in the new edition of Chambers's Encyc, should 

 be consulted. ' 



