404 GENERATIVE PROCESS IN PARAMECIUM. 



common Paramecium aurelia an example, which, although 

 exceptional in some particulars, affords peculiar facilities for the 

 observation of the process, and has been most completely studied 

 by M. Balbiani, it is here selected for illustration. This Ani- 

 malcule, as is well known, multiplies itself with great rapidity 

 (under favourable circumstances) by duplicative subdivision, which 

 always takes place in the transverse direction ; and the condition 

 represented in Plate xiv., figs. 1, 2, is not, as has been usually sup- 

 posed, another form of the same process, but is really the Sexual 

 congress of two individuals previously distinct. When the period 

 arrives at which the Paramecia are to propagate in this manner, 

 they are seen assembling upon certain parts of the vessel, either 

 towards the bottom or on the walls ; and they are soon found 

 coupled in pairs, closely adherent to each other, with their similar 

 extremities turned in the same direction, and their two mouths 

 closely applied to one another. The Paramecia and other free- 

 swimming Animalcules, while conjugated, continue moving with 

 agility in the liquid, turning constantly round upon their axes ; 

 but those which, like Stentor, are attached by a foot-stalk, remain 

 almost motionless (6g. 21). This conjugation lasts for five or six 

 days, during which period very important changes take place in 

 the condition of the reproductive organs. In order to distinguish 

 these, the Animalcules should be slightly flattened by compression, 

 and treated with acetic acid, which brings the reproductive appa- 

 ratus into more distinct view, as shown in Plate xiv., figs. 1-5. 

 In fig. 1 each individual contains an Ovarium, a, which is shown to 

 present in the first instance a smooth surface; and from this there 

 proceeds an excretory canal or oviduct, c, that opens externally at 

 about the middle of the length of the body into the buccal fissure, e. 

 Each individual also contains a Seminal Capsule, b, in which is 

 seen lying a bundle of spermatozoids curved upon itself, and which 

 communicates by an elongated neck with the orifice of the excre- 

 tory canal. The successive stages by which the seminal capsule 

 arrives at this condition from that of a simple cell, whose granular 

 contents resolve themselves (as it were) into a bundle of filaments, 

 are shown in figs. 6-10. In fig. 2 the surface of the ovary, a, is 

 seen to present a lobulated appearance, which is occasioned by the 



Spirostomum ambiguum), and even 100 or more (as in a species of 

 Urostyla), may be developed in a single individual. In some cases, 

 again, the subdivision does not involve the entire ' germ-cells ' in the 

 first instance, but affects only their ' germinal vesicles ; ' these being 

 multiplied in the midst of the undivided granular yolk-mass, but draw- 

 ing round themselves, near the time of conjugation, their several shares 

 of this substance, and becoming completed into ova by the formation of 

 an investment round their respective yolk-segments : this is the mode in 

 which ova are produced in the Vorticellina. In Paramecium it seems as 

 if the whole of the granular yolk-mass were not thus appropriated ; a 

 number of sterile yolk-segments (a, a, Plate xiv., fig. 5) being left after 

 the maturation of the ova. 



