268 Trof.E.K&rsten on the Formation, 



changed until the cellulose membranes are deposited upon its 

 outer and inner surfaces. 



Again, Mohl, Schleiden, and their followers supposed that in 

 this way a long-unaltered envelope is formed around the con- 

 tents, which are kept in constant change by continual diffusion; 

 whilst to me the phenomena of development appeared to indicate 

 a simultaneous and continuous chemical alteration both of the 

 membrane and contents of the cell. 



Having illustrated in that essay the manifold phases of deve- 

 lopment of the membrane of the secondary cells, and fully de- 

 scribed elsewhere (Abhandl. der Berl. Akad. 1847, p. Ill; 

 Botan. Zeitung, 1857, p. 314; Poggendorff's Annalen, 1860, 

 No. 4) the various chemical metamorphoses of the outer and 

 primary cell-membrane, consisting, at a certain stage of develop- 

 ment, of cellulose — changes which have been more fully esta- 

 blished by Wigand (Desorganisation der Pflanzenzelle, 1862) — 

 I shall here endeavour again to prove that all cells of vegetable 

 tissue, as far as observation has yet gone, originate as minute 

 free vesicles within the fluid contents of previously existing cells, 

 and attain their normal dimensions after undergoing many de- 

 terminate chemical changes ; and, moreover, that the involution 

 of the parent cell to form septa, as far as is yet made out, 

 though it may accompany its multiplication, does not originate 

 or cause it. 



Besides the tissue-cells which originate in the juices of the 

 cell, either singly for the maintenance of the individual, or 

 several together for the purpose of multiplication, there appear 

 in the cell-fluid generations of cells, originating and disappear- 

 ing in manifold sequence, which are I'ecognized as the producers 

 of the more composite organic compounds, and partly also as 

 the cause of the variety in the form of the thickening layers of 

 the originally structureless membrane of the tissue-cells to which 

 they adhere. 



That these non-nucleated vesicles (starch, chlorophyll, &c.), 

 spoken of by me as secretion-cells, are actual cell structures, 

 and not structureless corpuscles, is not merely a curious histo- 

 logical fact, as it was in a great measure regarded by the older 

 anatomists, but one of great impoi'tance to the physiologist; 

 for if an anatomist of note, speaking of the constant tendency 

 of anatomy towards the investigation of the most minute con- 

 ditions of organization, has asserted that physiology will sub- 

 side into a subtle anatomy, experience will soon show, on the 

 contrary, that physiology will rather rise in a subtle anatomy. 



In the following pages the nature of the origin and of the 

 growth of all the cells of the different tissues of the organism 

 cannot, necessarily, be demonstrated; for with many of them 



