152 FOOT-NOTES TO EVOLUTION. 
points of interest which are still a matter of contro-- 
versy. 
According to many earlier observers, and especially 
clearly enunciated by the pioneers of the cell theory, 
Schleiden and Schwann, it was generally assumed that 
cells arose by a sort of crystallization out of an unorgan- 
ized ground substance—the “cytoblastema” of Schlei- 
den. This idea, however, was soon overthrown by the 
more complete observations of von Mohl, Unger, 
Naegeli, Remak, Kélliker, and others, and the founda- 
tion was laid for the important generalization of Vir- 
chow, “ omnis cellula e cellula,”’ which has since become one 
of the most fundamental principles of 
biology. Every cell is derived from a 
pre-existing cell by a process of division, 
and this process has gone on unceasingly from the time 
when life first began down to the present moment. All 
life comes from pre-existing life, and, whatever may have 
in some past time occurred, the spontaneous production 
of living substance from a non-living condition does not 
now exist. 
One of the earliest results of the study of cell multi- 
plication was the discovery that division of the nucleus 
precedes the division of the cell body. Furthermore, a 
careful examination of the different phases of the pro- 
cess offers the strongest proof that the most important 
feature of this division, an end to which all the other 
processes are subsidiary, is the exact halving of a certain 
nuclear substance, the chromatin, between the two 
daughter cells which result from the division. To gain 
a clear conception of this process of in- 
direct cell division, or “ karyokinesis,” 
let us consider the changes which take place in typical 
cell multiplication. Two parallel series of changes oc- 
cur nearly simultaneously, the one affecting the nucleus, 
Cell multipli- 
cation, 
Karyokinesis. 
