THE VITAL PROPERTIES OF THE CELL 255 



divisions, without sexual intercourse (vegetative reproduction). 

 Or, to speak generally, between two acts of fertilisation a large 

 number of events, which are the result of cell division, are intro- 

 duced ; these, however, need not belong to a single highly 

 developed physiological individual, but may be shared by 

 numerous individuals. This may occur in one of two ways. 



In the one case the organism proceeding from the fertilised 

 egg is unable itself to form sexual cells ; it is only able to mul- 

 tiply by means of buds, spores, or parthenogenetic ova. These, 

 or their asexually produced descendants, then become sexually 

 mature, and develop the capacity of producing ova and sperma- 

 tozoa. Such a cycle of events is called a regular alternation of 

 generations (Hydroid polyps, Trematodes, Cestodes, partheno- 

 genesis of Aphides, Daphnids, etc. Higher Cryptogams). 



In the second case the organism derived from the fertilised 

 egg multiplies both sexually and asexually. The consequence of 

 this is, that even in the same species of plant or animal the 

 generation cycle must vary considerably. Between the comple- 

 tion of the first and the commencement of the second act of 

 fertilisation, either, on the one hand, only cell 'descendants arise 

 which belong to the single individual from which the fertilised 

 egg was derived, or one or more generations, the number in some 

 cases being very large, intervene, until finally the eggs of an 

 individual, produced by budding, become fertilised. In conse- 

 quence, fertilisation here assumes the character of a facultative 

 process, which is not absolutely necessary for the continuation of 

 the species, at any rate, so long as it has not been proved that 

 there are definite limits to vegetative multiplication. At present 

 this cannot be demonstrated in numerous plants, which appear to 

 be able to multiply indefinitely by means of runners, tubers, etc. 



When we consider such cases, we must admit that the vital 

 processes may continue indefinitely simply by repeated division 

 of the cells themselves, without the intervention of the act of fer- 

 tilisation ; still, on the other hand, we are bound to conclude, on 

 account of the wide distribution throughout the whole organic 

 kingdom of the phenomenon of fertilisation, that this institution 

 is of essential importance amongst the vital processes, and that it 

 is fundamentally connected with the life of the cell. Fertilisa- 

 tion is in fact a cellular problem. 



Our present subject is most closely connected with the study 

 of the eel), especially as concerns its irritability and divisibility. 



