* Vide an elaborate paper by Dr. H. Cienkowski, in the 'Bulletin de la Classe Physico-Mathematique de l'Acad. Imp. des Sciences de St. Petersbourg' for January 1855.

(104). The encapsulation of Ploesconia Charon, likewise witnessed by Mr. Carter, presented analogous phenomena. "The Plcesconiae had not appeared twenty-four hours when they began to assume a globular form by becoming shorter and shorter, and at the same time secreting an albuminous substance around them, within which the legs and cilia were gradually withdrawn, and a spherical capsule was thus formed, which adhered to the watch-glass. All trace of the Plcesconiae now became lost, with the exception of the refractive globules in the abdominal cavity; and the contracting vesicle, although active at first, soon disappeared. In this passive state of existence the Plcesconiae continued for two days, when the contracting vesicle again began to resume its functions, and the included animalcule, detaching itself from its capsule, began to rotate gradually. Cilia next presented themselves, the rotatory motion became accelerated, the distension of the contracting vesicle increased, until at last the capsule burst, and a portion of the cilia of the Ploesconia was at the same time protruded.

A few minutes of rest now succeeded; and the cilia having been withdrawn, the rotatory motion was again resumed, while the distension of the contracting vesicle also going on, at last became so great as to force a portion of the body of the encysted Ploesconia through the rent, when an interval of rest followed. The portion of the body was then withdrawn, and the same process repeated several times, until at length the Ploesconia obtained its exit. It was now almost spherical in shape, from the enormous distension of the contractile vesicle, behind which appeared numerous globules, and at the circumference the legs and cilia. In this state it continued stationary for some minutes, until the distensions of the contracting vesicle, evidently increased for the bursting of 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 gemmation which would assimilate them rather to bulbilli, or caducous buds, than to embryos originating from fertile ova. They would seem, however, to be really produced by an act of sexual generation. We will describe the phenomena witnessed by M. Balbiani in the common green Paramecium (Paramecium Bursaria,Focke; 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 generally 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 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 a sexual reproductive apparatus takes place.

(106). At the end of this time the nucleolus has undergone a considerable increase in size, and has become converted into a sort of capsule of an oval form, the surface of which 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, in a very irregular manner, and form so many secondary sacs or capsules. At a period which is still nearer that of division, these latter appear to be composed of an extremely fine membrane enveloping a number 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. It also contains a perfectly colourless and homogeneous fluid.

* "On the existence of a Sexual Reproduction in Infusoria," Comptes Rendus, 29th March, 1858, p. 628.

At the same time, the nucleus has also changed its form and aspect; it has become rounded and widened; its substance has become softer, lost its refractive power; and towards the margins it presents notches which, penetrating more and more deeply into its mass, isolate one or more fragments, in which a sufficient magnifying power enables us to see a certain number of small transparent spheres with an obscure central point. In other cases the nucleus while still almost entire presents this aspect, and then appears as if stuffed with these little rounded bodies, the analogy of which to ovules cannot be doubted in the least. The evolution of the nucleus and nucleolus being identical, and progressing at the same rate in the two coupled individuals, it follows, if we regard the former as an ovary and the second as a testicle or seminal capsule, not only that each of them possesses the attributes of both sexes, but that they fecundate each other.