CELLS. 41 



Most cells are too small to be distinguished except 

 through lenses ; many Protozoa, e.g., large Amoeba;, 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 ; 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 used 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 Ama-ba in 1755, and Fontana, in 17S4, discovered the kernel or 

 nucleus of the cell. 



But the fact that Bichat, in his Aiiatomie Gcncrale (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 Midler 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-g, Schwann and Schleiden 

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

 and develop from cells, thus estaljlishing the famous " cell theory," or, 

 rather cell doctrine : — " There is one universal principle of development 

 for the elementary part of organisms however different, and this principle 

 is the formation of cells." ^ 



This doctrine was corroborated in many ways. Numerous investi- 

 gators, Prevost and Dumas (1824), Martin Barry (1838-41), Reichert 

 1840), Henle (1841), KoUiker (1843-6), and Remak (1841-52), showed 

 how the cells of the embryo arise from the division of the fertilised 

 egg cell. 



Moreover, Goodsir in 1845, Virchow in 185S, proved that in all cases, 

 pathological as well as normal, cells arise from pre-existing cells, that 

 oiiinis cellula e cellula is a general fact of histology. 



There was a strong tendency, however, to attach too much import- 

 ance to the cell wall, and too little to the contained cell substance. The 

 all important protoplasm was not adequately appreciated. 



In 1835, Dujardin described the "sarcode"of Protozoa, and other 

 animal cells ; in 1839, Purkinje compared the substance of the animal 



1 Those interested in history should read the scholarly history of cell lore hy Sir 

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

 Microscopical Society (Edin. 1S90, also in Xatiire, 1890). See also Professor 

 M'Kendrick On the Modern Cell Theory i^mz. Phil. Soc, Glasgow, 1S88), also his 

 text-book of Physiology. The articles Mori^hology and Pi^otoplasm in the 

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

 be consulted. 



