436 M. Balbiani on Sexual Reproduction in the Infusoria. 



this mode of reproduction; but under the influence of conditions 

 of which we are still ignorant, the species propagates itself in a 

 very different manner, and in the midst of phjenomena far more 

 complex than those which preside over the multiplication by 

 fissiparity. In this new mode we shall see the actual anatomical 

 signification of the nucleus and nucleolus, the function of which, 

 if we except the division of the former of these two organs in the 

 act of spontaneous division, has hitherto been perfectly passive. 

 It is, in fact, at their expense that the male and female repro- 

 ductive elements which characterize this mode of propagation 

 are formed. 



When the period arrives at which the Paramecia are to pro- 

 pagate with concourse of the sexes, they are seen assembling 

 upon certain parts of the vessel, either towards the bottom, or on 

 the walls. The copulation is always preceded by certain pre- 

 liminaries which are very curious to observe, but upon which we 

 cannot dwell here. Soon they are found coupled in pairs, 

 adherent laterally and as it were locked together, with the simi- 

 lar extremities turned in the same direction, and the two mouths 

 closely applied to each other. In this state the two conjugated 

 individuals continue moving with agility in the liquid, and turning 

 constantly round their axis. There is nothing, before the copu- 

 lation, to announce the considerable changes which are about to 

 take place in the nucleus, and the nucleolus which accompanies 

 it. It is during the copulation itself, of which the duration is 

 "prolonged for five or six days or more, that their transformation 

 into sexual reproductive apparatus takes place. 



The nucleolus has undergone a considerable increase in size, 

 and has become converted into a sort of capsule of an oval form, 

 of which the surface presents longitudinal and parallel lines or 

 streaks. Nearly always, it soon divides in the direction of its 

 greater axis, into two, or more frequently into four parts, which 

 continue increasing independently of each other, and in a very 

 irregular manner, and form so many secondary sacs or capsules. 

 At a period which is still near that of division, these latter appear 

 to be composed of an extremely fine membrane, enveloping a 

 bundle of small, curved bacilla, extending from one extremity of 

 the sac to the other, inflated towards the middle, narrowed 

 towards the extremities. It is these which, when seen through the 

 enveloping membrane, give the capsule the striated appearance 

 which is characteristic of it, and which even exists in the nucleo- 

 lus at almost all the other periods of the life of the Infusorium. 

 It also contains a perfectly colourless and homogeneous fluid. 



At the same time the nucleus has also changed its form and 

 aspect ; it has become rounded and widened ; its substance has 

 become softer and lost its refractive power, and towards its mar- 



