As regards this fecundation itself, everything seems to prove that it takes place by means of an exchange made by the two coupled individuals of one or more of their seminal capsules, which pass through the apertures of their mouths, closely applied to each other, from the body of one Paramecium into that of the other; for very often, although we may not be able to perceive this passage itself, we may detect the moment when one of the capsules already engaged in one of the mouths is on the point of clearing this aperture.

(107). Each capsule after its transmission continues to increase in size in the body of the individual which has received it - frequently attaining a volume greater than that of the nucleus itself; but there is never more than one that arrives at maturity at the same time. When, having arrived at this state, it is examined after being pressed out of the body of the animalcule to free it from the granulations which mask it more or less while there, it appears under the form of a large ovoid body, the surface of which presents a multitude of parallel striae directed longitudinally, due to the arrangement of the corpuscles contained in the interior. Compression, carried so far as to cause its rupture, shows it distinctly to be formed by a membrane of extreme tenuity containing an immense number of minute fusiform corpuscles, the extremities of which are imperceptible on account of their extreme fineness. As soon as they are free, these little bodies exhibit a vacil-latory and translatory movement, which soon causes their dispersion in the circumambient fluid.

These are the spermatozoids of Paramecium Bursaria.

(108). It is usually from the fifth to the sixth day following the copulation that the first germs are seen to make their appearance, in the form of small rounded bodies formed by a membrane which is' rendered very evident by acetic acid, and containing a greyish, pale, homogeneous or almost imperceptibly granular matter, in which neither nucleus nor contractile vesicle is yet to be distinguished: these organs do not appear till afterwards. Stein and F. Cohn have shown that these embryos quit the body of their parent under the form of Acinetos, furnished with knobbed tentacles (true suckers, by means of which they remain for some time still adherent to the mother, deriving their nourishment from her substance); but their investigations did not reveal to them the ultimate fate of these young animalcules. M. Bal-biani was able to follow them for a considerable time after they had detached themselves from the parent animalcule, and convinced himself that, after losing their suckers, becoming clothed with vibratile cilia, and obtaining a mouth, which first shows itself in the form of a longitudinal furrow, they definitely acquired the form of the mother, becoming filled in the same way with the green granulations characteristic of this Paramecium, without undergoing any more important metamorphoses.

M. Balbiani* did not succeed in witnessing the deposition of the ova, but he thinks it very probable that they escape by the anus or by some neighbouring aperture. In Stylonychia he observed them to collect in the posterior part of the body and diminish gradually in number from the first or second day after copulation, at which period a round pale body begins to make its appearance in the centre of the animalcule, which becomes constricted in the middle and reconstitutes the double nucleus.

(109). The Infusoria would seem to be destitute of copulatory organs. In most cases sexual intercourse is effected by simple juxtaposition of the mouths of the two animalcules. In the Oxytrichina the union is more intimate, and goes so far as to constitute a true soldering of the two individuals for more than two-thirds of the anterior part of their length. Any one who had not witnessed all the phases of this remarkable copulation would necessarily regard this state as a case of longitudinal division proceeding from behind forwards; but the concomitant changes of the internal organs cannot leave the least doubt as to the signification of this act.

* " On the Generative Organs of the Infusoria," Ann. Nat. Hist. ser. 3. vol. ii. p. 443.

(110). The remarkable history of Trichoda Lynceus has been studied by M. Jules Haime*. This animalcule would appear to commence its existence under the aspect of an Oxytricha (Ehrenberg) (fig. 22,1,2), in which condition it undergoes multiplication by transverse fissure in the usual manner (3, 4.) The halves resulting from this division then assume a globular form, lose the greater part of their locomotive apparatus, and become encysted (5, 6.) The contents of the cyst next separate from their envelope, leaving a space on one side, in which ciliary movement can be distinguished (7.) This space spreads all round; granular matter is expelled from the cyst, whereby its form becomes altered (8), and the distinction between the newly formed animalcule to which the cilia belong and the remains of the original creature becomes more and more apparent (9,10,11), until at last the former escapes through the walls of the cyst (12, 13, 14, 15, 16) and soon developes itself into an Aspidiscus. How the Aspidiscus returns to the Oxytricha-form is still a matter for speculation.

Metamorphoses of Trichoda Lynceus.

Fig. 22. Metamorphoses of Trichoda Lynceus. 1. Larval condition (Oxytricha), seen from below. 2. The same, after swallowing another animalcule. 3. A very large specimen just about to undergo fission. 4. The same, showing the flssiparous process in a more advanced stage. 5. One of the products of such fission. 6. The same, having lost most of its cilia and assumed a spherical shape. 8. The same, now become completely motionless. 9. Appearance of the same after the lapse of fifteen days. 10. Later condition of the same, showing the formation of a cyst. 7 & 11. After the expulsion of a considerable quantity of exuvial matter, a ciliated animalcule appears in the interior of the cyst. 12 & 13. The included animalcule escapes from its encystment and at length assumes the form of an Aspidiscus, represented at 14. 15. The complete animalcule walking by means of its setae. 16. Inferior aspect of the same when at rest. - All the figures are magnified 355 diameters, except the two last, which are shown under a magnifying power of 760 diameters.

* Ann. des Sc. Nat. ser. 3. t. xix. p. 109.