The conception or idea connected with the word " cell," used 

 scientifically, has been considerably altered during the last fifty 

 years. The history of the various changes in this conception, or 

 the history of the cell-theory, is of great interest, and nothing 

 could be more suitable than to give a short account of this history 

 in order to introduce the beginner to the series of conceptions 

 connected with the word " cell " ; this, indeed, may prove useful 

 in other directions. For whilst, on the one hand, we see how 

 the conception of the cell, which is at present accepted, has 

 developed gradually out of older and less complete conceptions, 

 we realise, on the other hand, that we cannot regard it as final or 

 perfect ; but, on the contrary, we have every ground to hope that 

 better and more delicate methods of investigation, due partly to 

 improved optical instruments, may greatly add to our present 

 knowledge, and may perhaps enrich it with a quite new series of 

 conceptions. 



The History of the Cell-Theory. The theory, that organ- 

 isms are composed of cells, was first suggested by the study of 

 plant- structure. At the end of the seventeenth century the 

 Italian, Marcellus Malpighi (I. 15), and the Englishman, Grew (I. 

 9), gained the first insight into the more delicate structure of 

 plants ; by means of low magnifying powers they discovered, in 

 the first place, small room-like spaces, provided with firm walls, 

 and filled with fluid, the cells ; and in the second, various kinds of 

 long tubes, which, in most parts of plants, are embedded in the 

 ground tissue, and which, from their appearance, are now called 

 spiral ducts or vessels. 



Much greater importance, however, was attached to these facts 

 after the investigations, which were carried on in a more philo- 

 sophical spirit by Bahn towards the end of the eighteenth century, 

 were published. 



Caspar Friedrich Wolff (I. 34, 13), Oken (I. 21), and others, 

 raised the question of the development of plants, and endeavoured 

 to show that the ducts and vessels originated in cells. Above all, 

 Treviranus (I. 32) rendered important service by proving in his 

 treatise, entitled Vom inwendigen Bau der Gewachse, published in 

 1808, that vessels develop from cells ; he discovered that young 

 cells arrange themselves in rows, and become transformed, by the 

 breaking down of their partition walls, into elongated tubes ; this 

 discovery was confirmed and established as a scientific fact by the 

 subsequent researches of Mohl in 1830. 



