162 GENERAL RETROSPECT. 



figure, especially in animals ; in plants they are, for the most 

 part or exclusively, cells. This variety in the elementary 

 parts seemed to hold some relation to their more diversified 

 physiological function in animals, so that it might be established 

 as a principle, that every diversity in the physiological signi- 

 fication of an organ requires a difference in its elementary 

 particles ; and, on the contrary, the similarity of two elemen- 

 tary particles seemed to justify the conclusion that they were 

 physiologically similar. It was natural that among the \ery 

 different forms presented by the elementary particles, there 

 should be some more or less alike, and that they might be 

 divided, according to their similarity of figure, into fibres, which 

 compose the great mass of the bodies of animals, into cells, 

 tubes, globules, &c. The division was, of course, only one of 

 natural history, not expressive of any physiological idea, and 

 just as a primitive muscular fibre, for example, might seem to 

 differ from one of areolar tissue, or all fibres from cells, so would 

 there be in like manner a difference, however gradually 

 marked between the different kinds of cells. It seemed as if 

 the organism arranged the molecules in the definite forms 

 exhibited by its different elementary particles, in the way 

 required by its physiological function. It might be ex- 

 pected that there would be a definite mode of development 

 for each separate kind of elementary structure, and that it 

 would be similar in those structures which were physiologi- 

 cally identical, and such a mode of development was, in- 

 deed, already more or less perfectly known with regard to 

 muscular fibres, blood- corpuscles, the ovum (see the Supple- 

 ment), and epithelium-cells. The only process common to 

 all of them, however, seemed to be the expansion of their 

 elementary particles after they had once assumed their proper 

 form. The manner in which their different elementary par- 

 ticles were first formed appeared to vary very much. In 

 muscular fibres they were globules, which were placed together 

 in rows, and coalesced to form a fibre, whose growth proceeded 

 in the direction of its length. In the blood-corpuscles it was 

 a globule, around which a vesicle was formed, and continued 

 to grow ; in the case of the ovum, it was a globule, around 

 which a vesicle was developed and continued to grow, and 

 around his again a second vesicle was formed. 



