AUTHOR'S PREFACE. xvii 



particles and a cell differs only in degree from that which exists 

 between two cells ; so also the principle of development in the 

 latter can only then be similar, when it repeats itself in the 

 rest of the elementary particles. 1 therefore quickly asserted 

 this position also, so soon as I was convinced of the accordance 

 between the cells of cartilage and those of plants in this sense. 

 It now became easy to accommodate the principle which I 

 had laid down to the rest of the tissues, since the principle 

 itself had already made me acquainted with the law of their 

 development. Actual observation also completely confirmed 

 the conclusion which had been drawn with respect to the rest 

 of the tissues. It was not absolutely necessary that this 

 principle should recur in the elementary particles of vascular 

 tissues ; for since no independent vitality of the elements, 

 and therefore no diversity in the fundamental powers of 

 growth, was assumed in their case, so, without prejudice to 

 the principle, might they be subject to entirely different laws 

 of development. But slight as was the probability at the 

 commencement, that the principle could be carried out with 

 respect to them, observation soon showed that vessels do not 

 establish any essential difference in growth, but merely occa- 

 sion some distinctions, which may be explained as the con- 

 sequences of a more minute distribution of the nutrient fluid; 

 of the change of material facilitated both by that means and 

 by the circulation ; and of a greater capacity of imbibition 

 in the animal substance. Thus was the proposition firmly 

 established by observation, that there is one common principle 

 of development for the elementary particles of all organised 

 bodies. It had already indeed been long known that all 

 tissues were formed from a grauulous mass; but that these 

 granules bore some direct relation to the subsequent ele- 

 mentary particles, and what that relation might be was known 

 in respect to but a few of the particles, and in them the mode 

 of development appeared to differ so much, that unity neither 



