CHAPTER III. 



REPRODUCTION AND DEVELOPMENT. 



Origin of cells : Every cell is produced by a preexisting cell 

 omnis ceUula e celhila. A generation ago it was supposed 

 that under certain circumstances some of the lower organisms, 

 as the bacteria, could develop directly out of non-living mat- 

 ter. The generation of life de novo in such a manner was 

 called abiogenesis, or spontaneous generation. The incorrect- 

 ness of that supposition has been shown, and it is now uni- 

 versally believed that abiogenesis never occurs. Under all 

 circumstances life is always produced by preexisting life 

 omne vivurn e vivo; all protoplasm is elaborated by preexisting 

 protoplasm. 



Modes of reproduction of cells : There are several modes in 

 which cells reproduce themselves, namely : 1, direct division ; 

 2, budding, or gemmation ; 3, segmentation ; 4, indirect di- 

 vision, or karyokinesis. 



FIG. 2. 



Direct cell-division (Flemming). 



Direct division : In this mode of cell-division a constriction 

 appears in the nucleus and in the cell-body ; this constriction 

 gradually deepens, until the two parts become completely 



3 Hist. 33 



