156 PHYSICAL CHEMISTRY 



siderable period. This rhythm must 'be due, therefore, to 

 metabolic changes, in the same way that the rhythm of the organs 

 of the human body is maintained. Various physical models have 

 been made showing periodicity, perhaps the most interesting of 

 which are the periodic catalysis mechanisms of Bredig and his 

 pupils. Under certain conditions mercury causes the decomposi- 

 tion of H 2 2 , and this in turn causes a film of oxide to form 

 on the mercury. In these models the film breaks periodically 

 and is reformed. If a crack appears in the film the potential 

 difference between mercury and oxide dissolves the film elec- 

 trolytically. The surface tension of the mercury differs from 

 that of the oxide, so that surface tension movements as well as 

 catalysis are produced (see Bredig, 1907, B. and Fajans, 1908, 

 B. and Fiske, 1912, B. and Weinmayr, 1903). 



Cell Division 



Cell division in animal cells seems to be a special form of 

 ameboid motion, and many cells show irregular ameboid move- 

 ments immediately before division. It is thought by some in- 

 vestigators that the division of the nucleus is necessary before 

 division of the cytoplasm can take place. On the contrary, it 

 was found that the cytoplasm of cells from which the nuclei 

 were removed is capable of division (McClendon, 1908). The 

 unfertilized starfish egg may be caused to divide by immersing 

 it in sea water charged with C0 2 , as will be described later. The 

 nuclei (maturation spindles) were removed from starfish eggs 

 and immersed in carbonated sea water, developing imperfect 

 mitotic figures (cytasters) and dividing into spherical masses of 

 protoplasm. In the parthenogenetic frog's egg, to be described 

 later, the cytoplasmic division commences an hour or more be- 

 fore the nuclear division commences, seeming to indicate that the 

 nuclear division is not the direct cause of the cytoplasmic division. 



Biitschli, Rhumbler and others considered cell division to be 

 the result of changes in surface tension. The constriction of 

 the cytoplasm follows a band of relatively high surface tension. 

 There seems to be no way of determining whether the surface 

 tension is increased at the equator or reduced at the poles, but 

 the division is often preceded by a rounding up of the cell, which 

 is probably the result of a general increase in surface tension. 



