io6 PERIOD IV. 



long after they have been separated from the body. 

 The preponderating importance of the transparent jelly 

 or protoplasm became clear when it was recognised 

 that this alone is invariably present, and that this alone 

 responds to stimuli. The nucleus is believed to be only 

 a specialised part of the cell-protoplasm. 



The cell-theory, like nearly every theory, was neither 

 altogether new nor in its first form altogether complete. 

 Before 1838 cell-division, as we should now call it, had 

 been indistinctly seen to be the process by which the 

 body of one of the higher animals is built up. Leeuwen- 

 hoek and Swammerdam had found a wholly cellular 

 stage in frog-embryos (see p. 103), while Prevost and 

 Dumas in 1824 had in effect discovered that the cells of 

 which such embryos consist result from repeated division 

 of an egg ; Mohl in 1835 observed the actual division. 

 Even Schwann, however, was not acquainted with the 

 important fact that every cell arises by the division of a 

 pre-existing cell. 



Swarm-spores of algae showed that protoplasm, when 

 unenclosed in a cell-wall, can move about, direct its 

 course, and change its shape. Knowledge of this fact 

 did more than rectify the definition of the cell ; it effaced 

 one distinction between plants and animals, and gave a 

 hint of the resemblance of primitive cells to such simple 

 organisms as Amoeba. 



Martin Barry in 1843 announced that certain Protozoa 

 (that name was not yet in use) are simple cells. He 

 pointed out that they possess nuclei, like those of tissue- 

 cells, and compared their increase by fission with the 

 cleavage of the egg. Single cells were thus shown to 

 be not only capable of locomotion, which was already 

 known, but able to provide for their own support. The 

 Protozoa and Protophyta (i.e., the simplest animals and 



