THE VITAL PROPERTIES OF THE CELL 267 



divide, but die, as a result of the changes induced by age, or, 

 as Maupas has expressed it, of senile degeneration. In order to 

 maintain the species, it seems to be absolutely necessary that 

 after definite periods two individuals should unite together in a 

 sexual act. In cultures such acts occur simultaneously through- 

 out the colony, so that a conjugation epidemic may be said to occur 

 occasionally. 



During an epidemic, which lasts for several days, the observer 

 sees hardly any isolated Infusoria in the culture glass, for they 

 are nearly all joined together in pairs. Maupas states that con- 

 jugation occurs in Leucophrys patula in the 300th generation, in 

 Onychodromus in the 140th, and in Stylonichia in the 120th genera- 

 tion. By a diminution of the amount of nourishment, the onset of 

 an epidemic may be hastened; by an increase it may be postponed, 

 or even permanently prevented, in which case the individuals 

 perish from senile degeneration. 



If, after these preliminary remarks, we examine more closely 

 the process of fertilisation, we find that, during a period of several 

 days, the following peculiar and interesting changes take place 

 in the couples of Infusoria. We will take as the basis of our 

 description the Paramsecium caudatum, for, since it possesses but 

 one nucleus and one single paranucleus, it presents simpler condi- 

 tions than those seen in most other species (Fig. 148). 



When the inclination for conjugation arises, " two paramaecia 

 come close together ; at first only their anterior ends touch, but 

 later on their whole ventral surfaces are in contact, their mouth 

 openings being opposite to one another " (Fig. 148 I, o). An ir- 

 regular thickening develops over a small area in the neighbourhood 

 of these latter, if conjugation lasts for any considerable period. 

 Meanwhile the nuclear apparatus, including both the chief nucleus 

 and the paranucleus, has undergone fundamental changes. 



The chief nucleus becomes somewhat enlarged, its surface being 

 at first covered with protuberances and depressions (Fig. 148 II 

 IV, k); these protuberances extend themselves into longer pro- 

 cesses, which later on become separated off, and then gradually 

 split up into still smaller pieces (V, VI, k). Thus the whole chief 

 nucleus becomes broken up into several small segments, which 

 distribute themselves all over the body of the Infusorian (VII), 

 and finally become dissolved and absorbed. In a word, the main 

 nucleus, having played its part, becomes completely disintegrated? 

 during and after conjugation. 



