47 



phores and nuclei ; whilst, within the nucleus itself, it is only by growth 

 and division that any increase in the number of the chromosomes is pos- 

 sible. The same automatic mode of multiplication must necessarily 

 also take place in the minute physiogical units of which the plasma is 

 composed. 



The protoplast is thus able to retain its specific character, and has also 

 the power to form from living or dead material new organs of secondary, 

 or even primary, importance. 



The cell-wall, for example, is a product of plasmatic activity, and 

 is not formed by the growth or division of a pre-existent wall. Other 

 permanent organs, and transitory ones as well, may be produced partly 

 or entirely from living substance. Just as an embryonic tissue may 

 according to circumstances develop different organs, so also may cilia 1 arise, 

 under certain circumstances, from the ectoplasm of a myxomycete, as new 

 formations not directly inherited from the parent. The membrane which 

 bounds the protoplast externally, and surrounds every vacuole, has a 

 special functional importance as the limiting membrane of the plasma. 

 Vacuoles, also, are organs which may originate where none previously 

 existed, but which, when once formed, may increase in number by division. 

 In certain cases the vacuoles appear necessarily to increase in - number 

 by a process of division of pre-existent vacuoles, and similarly the 

 cell-wall persists from generation to generation of yeast-cells, though 

 it may also undoubtedly be formed where none was previously existent. 

 Like the cell-wall, vacuoles may serve a variety of purposes, and under 

 particular conditions may be absent. It is not at all surprising that non- 

 living material may be interposed between the living substance which 

 regulates the vital mechanism as a whole, and that such non-living sub- 

 stance may have most important and necessary functions to perform. 



Moreover it can hardly be doubted that particular ends may be 

 attained by a specific grouping or arrangement of certain living physio- 

 logical units, while, in the same way, a formation of new and special 

 protoplastic organs is conceivable. Indeed it is not conclusively deter- 

 mined as yet whether nucleoli and centrosomes are preformed organs 

 which reproduce by division, or whether they may appear at one time 

 and disappear at another 3 . 



It is quite erroneous to suppose that each particular organ serves 



1 On cilia, see Zimmermann, Beihefte z. Bot. Centralbl., 1894, Bd. iv, p. 169; A. Fischer, 

 Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 156. 



2 Pfeffer, Zur Kenntniss d. Plasmahaut u. Vacuolen, 1890, p. 224. See sections 3 and 18 of 

 this book. 



s See Strasburger, Jahrb. f. wiss. Bot., 1895, Bd. xxvin, pp. 151, 172 ; 1897, Bd. xxx, pp. 379, 

 387; R. Hertwig, Centrosom u. Centralspindel, 1895, as well as the literature here cited; Zimmer- 

 mann, Zellkern, 1896; Boveri, Zur Physiol. d. Kern- u. Zelltheilung, 1897. 



