APPENDIX. - On the relations which exist between the aninual and vegetable kingdoms, as regards the function of reproduction. In the introduction to the foregoing article, it was observed that, if any analogies in deve lopment may be supposed to exist between plants and animals, they are to be sought be tween the lowest members of the two series. Whether we conclude that it is or is not pos sible to mark out the limit which separates the one kingdom from the other, it is not to be overlooked that the phenomena of reproduc tion, and consequently the whole circle of the development, of the zoosporous Algee resem ble more those which present themselves on the other side of the disputed territory, than those which occur among the higher plants. Let us compare the development of a unicel lular Alga, with that of one of the simplest Infusoria. An egg-shaped body composed of a homogeneous and contractile substance — as regards its chemical constitution nitroge nous—displays active motions, and exhibits two locomotive organs springing from its smaller end. Soon, however, its motions be come languid ; a newly formed cellulose mem brane, which is not contractile, encloses it, and now it undergoes a kind of cleavage, which results in the formation of a number of new bodies. In each of these, as soon as they escape from the parent, the same transforms tion is repeated.
In the other case, taking the development of Vorticella as an illustration (in the de scription of' which we follow Steino), we find that a disc-shaped mass of homogeneous con tractile substance (a monad), is transformed into a stalked and ciliated Vorticella. After having been for a time endowed with ac tive 'notion, and with a power of ingesting food, the Vorticella enters into a state of re pose, and at the same time is enclosed in a flexible membrane or cyst. ' The interior of the cyst is now occupied by a mass of proto plasma, which is no longer contractile, and presents no trace of the structure of the former Vorticella. By a process similar to that which occurs in the plant, this plasma divides into a number of disc-shaped bodies, resembling that from which the parent origi nated.
Between the Protozoon and the Protophy ton, there is an intermediate group, of which the Euglena viridis, alluded to in § I., may be
considered as the representative. The Eu glena after actively moving for a tirne, enters into the condition of repose, becoming at the same time enclosed in a new membrane. What follows this change, however, has not been as yet ascertained.
The phenomenon of conjugation, also, while it is without parallel among the higher plants, presents itself under nearly sitnilar conditions among the Infusoria. According to the ob servations of Stein, the circle of changes described in the preceding paragraph, is not the only one by which in Vorticella the spe cific form is reproduced. A Vorticella enters into a state of rest, and becomes encysted ; it is not now, however, converted into a mass of homogeneous protoplasma as in the former case. The cyst membrane changes into a thin walled vesicle, while from the body of the enclosed Vorticella, which has assumed a spherical form, there emanate a number of contractile radiating processes. It is now a Protozoon, identical with that to which has been given the name Actinophrys. Now in Actinophrys, the occurrence of conjugation has been recorded by several trustworthy observers. It was first described by Kiil liker#, afterwards by Siebold t, and finally by Cohn. t According to the last-mentioned author, two neighbouring individuals after approaching more and more closely to each other, emit from their opposite surfaces, vesi cular processes, which finally unite. As the union becomes more complete, the two seem to fortn but a single animal. As to what are the results of this remarkable conjugation, neither Cohn, nor, as far as we know, any other observer, is able as yet to speak posi tively. Every fully formed Actinophrys ex hibits embedded in its substance a central nucleus-like body ; this nucleus, according to Stein, is sooner or later transformed into an egg-shaped animal, which grows at the expense of the parent, and finally becomes endowed with active motion. At the smaller end is formed a crown of cilia, at the larger an oral depression, and soon there presents itself a perfect Vorticella. It is, at least. extremely probable that this development is the result of the previous conjugation of two Actino phries.