XVi AUTHOR’S PREFACE. 
in their physiological signification; and the diversity m the 
laws which it was necessary to assume in the development of 
a cell and a fibre, was also, only in a less degree, necessarily 
assumed between the different kinds of cells and the different 
sorts of fibres. Cells, fibres, &e. were therefore merely natural- 
history ideas, and no conclusion could be drawn from the mode 
of development of one kind of cell as to that of any other kind ; 
and, in fact, no such deductions were made, although we were ac- 
quainted with some important points in the process of develop- 
ment of certain kinds of cells ; for example, the blood-corpuscle 
(see p. 67 of this Treatise), and the ovum (see the Supplement, 
p. 217). Although the investigations quoted above determined 
the important fact of the non-vascular growth, they did not 
thereby effect any change in our views. The idea of proving 
the similarity of the principle of development for elementary 
particles which were physiologically different, by a comparison 
of animal cells with those of vegetables, was not contained in 
those researches, and with these, therefore, the mvestigators 
before mentioned might well come to a stand-still. 
The discoveries of Schleiden made us more accurately ac- 
quainted with the process of development in the cells of plants. 
This process contained sufficient characteristic data to render 
a comparison of the animal cells in reference to a similar 
principle of development practicable. In this sense I com- 
pared the cells of cartilage and of the chorda dorsalis with 
vegetable cells, and found the most complete accordance. The 
discovery, upon which my inquiry was based, immediately lay 
in the perception of the principle contained in the proposition, 
that two elementary particles, physiologically different, may 
be developed in the same manner. For it follows, from the 
foregoing, that if we maintain the accordance of two kinds of 
cells in this sense, we are compelled to assume the same princi- 
ple of development for all elementary particles, however dis- 
similar they may be, because the distinction between the other 
