THE VITAL PROPERTIES OF THE CELL 293 



If single individuals of Onychodromus, of a generation between the 

 140th and the 230th, or specimens of Stylonichia pustulata of a 

 generation between the 130th and the 180th, do not have the op- 

 portunity of pairing, they become old sexually, or over-mature. 

 It is true that they continue to multiply by means of division, and 

 indeed are able to pair, but no result is produced. For, in spite of 

 their pairing, they degenerate and succumb to a gradual decay of 

 their organisations, as Maupas expresses it, " in consequence of 

 senile degeneration." The commencement of this stage may be 

 recognised by characteristic changes in the nuclear apparatus. 



Swarm-spores or gametes of Alga? often die off, after swimming 

 about in the water for a few hours, without having succeeded in 

 pairing with suitable individuals. The receptive capacity of the 

 large female gamete of the species Cutleria, after it has come to 

 rest, and has become capable of functioning as an egg, only lasts 

 for a comparatively short time. Falkenberg (VII. 10) has per- 

 formed a large number of experiments which show " that, whilst 

 on the third day after they have come to rest almost all the eggs 

 are capable of becoming fertilised, on the fourth day only half 

 are in that condition. Further, after this period all the eggs lose 

 their receptive capacity, and although spermatozoids are placed in 

 their neighbourhood, commence to die off, exhibiting the same 

 changes as those eggs which were completely shut off from the 

 fertilising cells." 



Finally, mature animal egg-cells, even when under normal con- 

 ditions in the ovary or in the oviducts, live only for a short time ; 

 they soon become over-mature (Hertwig VI. 32). Their normal 

 functions become weakened, as is seen by the fact that, although 

 they can still undergo fertilisation for a time, yet this occurs in 

 an abnormal fashion ; several spermatozoa make their way into 

 the egg, the result being an abnormal process of development. 

 Without doubt, this phenomenon is analogous to the senile de- 

 generation of Ciliata which have been prevented from pairing at 

 a suitable period. 



The third law, that the commencement of the need for fertilisa- 

 tion may be hastened or postponed by external circumstances, 

 may be clearly proved in some cases. 



Thus, if nourishment be continually and abundantly supplied to 

 cultures of Ciliata, pairing can be prevented (Maupas VII. 30). 

 They continue to divide until the whole culture dies off in con- 

 sequence of senile degeneration. On the other hand, cultures of 



