AUTHOR'S PREFACE. xv 



In the above-mentioned investigations of Henle, Turpin, and 

 Dumortier, the resemblance whicli the animal tissues examined 

 (epithelium and the liver or yelk of snails) bore to plants, lay, 

 in the first place, in the circumstance, that their elementary 

 particles grew without vessels, and in part, free in a fluid, or 

 even inclosed in another cell ; and in the second place, in that 

 these elementary particles exhibiting a non-vascular growth, 

 were furnished with a peculiar wall, like the cells of plants. 

 When this coincidence was furnished, we were entitled to 

 arrange these cells as near to the vegetable cells as the different 

 kinds of animal cells, for instance, germinal vesicles, blood-cor- 

 puscles, and fat-cells, stand together, when regarded as different 

 species comprised under the natural-history idea of cells. 



The state of the matter, therefore, when I commenced my 

 researches was as follows : The elementary particles of or- 

 ganised bodies presented the greatest variety of form; there 

 was a resemblance between many of them, and, according to 

 the greater or lesser degree of similarity, a group of fibres, of 

 cells, of globules, and so on, might be distinguished, and 

 in each of these divisions again there were different forms. 

 As the cells taken collectively differed from the fibres, so also, 

 only in a less degree, must the separate kinds of cells differ from 

 each other, and the different kinds of fibres from each other. 

 All those forms seemed to have nothing else in common, save 

 that they grew by the addition of new molecules between those 

 already existing, that they were living elements. So long as 

 the epithelium-cells were regarded as a secretion of the 

 organised substance, they could never, in that sense, be classed 

 with the living elementary particles. There seemed to be no 

 general rule with respect to the mode in which the molecules 

 were joined together to form the living particles ; here they 

 united into one kind of cells, there into another, and at a third 

 spot into a fibre, and so on. The principle of development ap- 

 peared to be altogether different for such particles as differed 



