50 INFUSORIA. 



the capsule, were gradually reduced to their natural limits, when the 

 different parts of the Ploesconia regained their respective positions, and 

 the animalcule at last assumed its original form and bounded off in 

 search of food." 



(105.) The discovery of the propagation of the Infusoria by means 

 of embryos or internal germs has opened a new field of research in the 

 history of the development of these animalcules. From the researches 

 of M. Balbiani *, it would appear that, besides the truly agamic modes 

 of reproduction, namely, by spontaneous division and gemmiparity, 

 there exists a process by which the young are formed in the interior of 

 the parent, which gives them birth by the agency of distinct sexual 

 apparatus. Stein was one of the first to call attention to the part played 

 by the nucleus in the process of reproduction ; but he thought that the 

 germs were developed on the surface of this body by a species of gem- 

 mation which would assimilate them rather to bulbilli, or caducous buds, 

 than to embryos originating from fertile ova. They would seem, how- 

 ever, to be really produced by an act of sexual generation. We will 

 describe the phenomena witnessed by M. Balbiani in the common green 

 Paramecium (ParameciumBursaria,l?0(ke; Loxodes Bursaria, Ehrenb.), 

 as an illustration of what occurs in a great variety of other forms. 



In this species, as probably in all Infusoria, there exists a nucleus, 

 which is here accompanied by a small lenticular body, usually lodged 

 in an excavation of the nucleus near one of its extremities, and gene- 

 rally described under the improper name of " nucleolus." 



For several generations the Paramecia multiply by spontaneous 

 scission, each of the two new individuals obtaining half of the primitive 

 nucleus ; but under the influence of conditions of which we are still 

 ignorant, the animalcule is propagated in a very different manner. 

 When the period arrives at which the Paramecia are to propagate with 

 concourse of the sexes, they may be seen assembling upon certain parts 

 of the vessel in which they are contained, either towards the bottom or 

 on the walls. Soon they are found coupled in pairs, adherent laterally, 

 and as it were locked together, with the similar extremities turned in 

 the same direction, and their 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 this copulation to indicate the considerable changes that 

 are about to take place in the nucleus and the nucleolus which accom- 

 panies it. It is during the copulation itself (of which the duration is 

 prolonged for five or six days or more) that their transformation into 

 a sexual reproductive apparatus takes place. 



(106.) At the end of this time the nucleolus has undergone a con- 

 siderable increase in size, and has become converted into a sort of cap- 



* " On the existence of a Sexual Reproduction in Infusoria," Comptes Rendus, 

 29th March, 1858, p. 628. 



