CHAPTER VII. 

 THE VITAL PROPERTIES OF THE CELL. 



V. Phenomena and Methods of Fertilisation. Cell 

 reproduction by means of cell division, such as is described in 

 Chapter VI., does not, at least for the majority of organisms, ap- 

 pear to be able to continue for an indefinite period ; the process of 

 multiplication, after a shorter or longer period, comes to a stand- 

 still, unless it is stimulated afresh by the excitatory processes, 

 which are grouped together under the name of fertilisation. Only 

 the very lowest organisms, such as fission fungi, appear to be able 

 to multiply indefinitely by repeated divisions; for the greater 

 part of the animal and vegetable kingdoms the general law may 

 be laid down, that after a period of increase of mass through cell 

 division a time arrives when two cells of different origin must 

 fuse together, producing by their coalescence an elementary 

 organism which affords the starting-point for a new series of 

 multiplications by division. 



Hence the multiplication of the elementary organism, and with 

 it life itself, resolves itself into a cyclic process. After generations 

 of cells have been produced by division, the life-cycle returns to 

 the same starting-point, when two cells jnust unite in the act of 

 fertilisation, and thus constitute themselves the foundation of a 

 new series of generations. Such cycles are termed generation 

 cycles. They occur in the whole organic kingdom in the most 

 various forms. 



In unicellular organisms, for instance, the generation cycle 

 consists of a large number of independent individuals, which 

 sometimes amount to thousands. The fertilised elementary 

 organism multiplies by repeated divisions, producing descendants, 

 which do not require fertilisation, until a period arrives when a 

 new generative act occurs between the generations which have been 

 produced asexually. These phenomena have been most carefully 

 observed in Infusoria. Thus Maupas (VII. 30, p. 407) has proved 



