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, Kolliker, and others, and the founda- 

 tion was laid for the important generalization of Vir- 

 chow, " om/iis cellula e cellula," which has since become one 



of the most fundamental principles of 

 Cell multipli- , ■ , -^ n • j • j r 



^ biology. Every cell is derived from a 



cation. *'r . ,, , 



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- 



Karyokinesis. ,. ,,,... , , , • • ,. 



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, 



