In both cases the embryo is formed from the yolk or opaque central part of the ovum, by a series of changes sufficiently well known ; it is surrounded by a clear fluid, which is inclosed between it and the external pellucid membrane of the egg. By means of a lens, minute bodies may generally be perceived floating in the fluid, which by their motion serve to indicate the currents that take place in it; but with a little care the embryo may be extracted from the egg, and then the course of the currents along its surface can be ren dered more evident by the usual means. A (fg.310) is an enlarged view of the embryo of the Frog at the earliest stage at which I have detected the motion. The vertebral canal is just closed, and at the fore part of the body three ridges on each side indicate the com mencement of the gills. The arrows point out the course of the currents. They proceeded backwards along the dorsal surface, diverging in a direction downwards and backwards on the sides. They were visible but weaker on the abdominal surface. B represents the em bryo farther advanced, the currents have nearly the same direction but are better marked, they are strongest on the lateral eminences of the head which correspond to the future gills. In the embryo of the Newt, the phenomena are in a great measure similar ; the currents seemed, however, to begin and to continue most vigorous on the abdominal surface ; they are more particularly described in the paper re ferred to.
On extracting the embryo of the Frog, and viewing its surface in profile with Wollaston's doublet, moving cilia may be perceived on various parts. They appear like a transparent undulating. line on the surface, and, though very minute, are so distinct as to leave no doubt of their existence.
No one can fail to perceive the analogy which subsists between the phenomena just described, and those which occur in the ova of Zoophytes and Mollusca. I have not been able distinctly to perceive a rotation of the embryo of the Batrachia, as observed in the other instances, but Purkinje and Valentin State that they have seen it, and Rusconi ob served that the embryo of the Frog, when extracted from the ovum, turned round in a certain direction, which motion he supposed to be produced by water entering and issuing through pores in the skin.4' The phenomena in the Batrachian larvw have since been observed by Miiller,t Raspail,t and Purkinje and Valentin.§ The last men tioned naturalists also disting.uished the cilia and perceived the motion within the egg.
Adult Batrachia.—The ciliary motion was discovered in the adult Batrachia by Purkinje and Valentin ; indeed, it may not be improper again to state that the discovery of the phe nomena in adult Reptiles generally, and in Birds and Mammiferous animals, is due to these phy siologists.
According to their account, the ciliary mo tion in the Batrachia, as well as in all other vertebrated animals in which they have dis covered it, occurs in two situations within the body, viz. on the lining membrane of the
respiratory organs and on that of the genital organs of the female. They state that it exists over the whole internal surface of the lungs, and in the nose, mouth, and pharynx, extend ing as far back in the throat as the glottis, but no farther. They say nothing of the direction of the impulsion. Again, in the female, they discovered the motion on the internal surface of the oviduct. The result of my own ex amination of the Newt, Frog, and Toad is somewhat different. In all the three I found the ciliary motion very distinct in the mouth, throat, and gullet; in none could I perceive it in the lungs., notwithstanding very careful trials. In regard to the oviduct I have examined it only in the Newt, and although I could per ceive something like the motion on the edges of its superior orifice, I could not detect it on the internal surface of the tubell The ciliary motion in the mouth and throat occurs all the way from the opening of the mouth to the termination of the cesophagus. Its extent and the direction of the impulsion are easily ascertained by means of powdered charcoal ; they are pointed out by the arrows in the adjoining figures, A and B (fig. 311), which are taken from the Newt, the ap pearances in the Frog and Toad being not ma terially different. a is the lower jaw detached from the head, b the tongue, c the glottis, d the cesophagus cut off from the head (at g, g, fig. B), and laid open from above, e the sto mach, and f,f, the lungs. The general course of the impulsion, or, if in this case we might so express it, the currents, is longitudinal; they begin at the syrnphysis of the lower jaw and extend to the lower end of the cesophagus, where they terminate abruptly at the entrance of the stomach, thus differing from the de scription given by Purkinje and Valentin ; but it is worthy of notice that these observers de scribe the motion in the Tortoise and Serpent as extending the whole length of the cesopha gus. At particular parts the impulsion fol lows tbe direction of the plaits of the lining membrane. Figure B represents the head and the roof of the mouth, from which the lower jaw has been separated. On this part of the mouth also the general course is longitudinal, from before backward§; at the nostrils h, h, the particles are drawn in at one edge and issue at the other, as indicated in the outline of figure B.
As regards the use of the ciliary motion on the internal membranes of the Batrachia, we can scarcely doubt that one purpose is to convey onwards the secretions of these mem branes in the direction indicated. It is not impossible also that it may have some more intimate connection with the respiratory pro cess; but on this point we have not as yet suf ficient grounds for forming a probable opinion.