The phenomenon has been sought for in other parts of the body, but hitherto without success. Purkinje and Valentin state that on examination they could not find it in the fol lowing parts of vertebrated animals, viz. the skin, serous membrane, the alimentary canal, (except the mouth and gullet .of Reptiles,) the gall-bladder, the biliary and pancreatic ducts, the urinary organs, the seminal vesicles and ducts, the conjunctiva, cornea, and iris, the internal surface of the bloodvessels, the globules of the blood and lymph, the chorion, amnion, allantois, and yolk-sac of Birds. I have also repeatedly examined the fcetal membranes of the common Fowl, and with the same result.
3. Of the ciliary motion in the embryo.— According to Purkinje and Valentin the ciliary motion of the genital mucous membrane does not appear in the fcetus, nor until the animals have made some approach to the adult state ; that of the respiratory passages on the other hand becomes apparent in the embryo long before it attains maturity. The ciliary motion, however, to which I.ve would here refer is that which occurs at a much earlier period on the surface of the embryo of many animals, and generally causes it to perform a rotatory move ment within the ovum. It has now been ob served in the ova of Batrachia, Mollusca, Ac tiniw, Polypi, Sponges, and Infusoria. While the embryo is contained within the ovum, the cilia produce a current in a certain direction along its surface, or cause the whole embryo to move in the opposite direction ; hence the very remarkable rotatory motion which occurs in many instances, and which is so well marked in the Snail. When it has escaped from the egg, the embryo moves about in the water by means of the cilia, as happens also 1.vith the naked gemmules of the Sponge after they are discharged from the parent. The ciliary mo tion is subservient to the respiration of the embryo, by renewing the contact of the water or fluid contained ,in the .egg on the respiring surface, and in some .instances, the Mollusca for example, the motion is observed to be especially strong at the part where the respira tory organ is afterwards developed. When the embryo quits the egg; the cilia serve also for locomotion, and by this provision the gem mules of fixed zoophytes are disseminated, and conveyed to situations suitable for their future growth.
4. Figure, structure, and arrangement qf the, cilia in general.—The cilia are best seen when their motion slackens ; their shape, size, arrangement, and manner of moving may then be distinguished with tolerable accuracy, at least in the larger sort. Their figure is in
general that of slender, conical, or sometimes slightly flattened filaments, broader at the base or root, and tapering gradually to the point. Their size differs greatly' on different parts even of the same animal, but on corresponding parts of different individuals of the same species their size seems to be the same. The largest I have measured are those on the point or angle of the branchial laminw in the Bac cinum undatum ; they are at least 3-6 of an inch long. I have not attempted to determine the exact size of the smallest, but Purkinje and Valentin state it at 0.000075 of an inch, while they make the largest they have met with only 0.000908 in., which is considerably less than I have found them ; but they had no opportunity of examining marine animals, in which, gene rally speaking, the largest cilia are met with. In the Sea-mussel the darker-coloured cilia are about "'073 of an inch long, the others consider ably less.
The cilia are very generally arranged in re .gular order. In some cases they are placed in stmight rows, as on the gills of the Mussel; in others they form circles or spiral lines, as in many Infusoria; and Purkinje and Valentin state that in animals of the higher orders the .most prevalent mode of arrangement is in spiral Jines or ridges. They are generally set close together in the same row ; on the gills of the Sea-mussel I find there are seven or eight of the larger cilia in the length of Tko of an inch, or about seven or eight thousand to the length of an inch, but in other cases there are many more. In some instances they are erect, or at right angles to the surface on which they are planted, in others inclined, and then it would seem that the inclination is in the direction of the currents which they produce. In some parts they are straight, in others curved, not only when in action, but also when at rest, and the points are bent in the same direction in which the currents flow.
The substance of the cilia is transparent, and for the most part colourless; in some, however, it is coloured brown or yellowish brown. lt appears as if homogeneous, even when highly magnified, and no fibres or globules are distin guishable in it. It seems to vary somewhat in consistency, for the cilia on some parts appear extremely soft and pliant, and on others com paratively firm and elastic, though still abun dantly flexible.