« ПредыдущаяПродолжить »
The Phenomena of Reproduction in the Animal World.
THE faculty of giving existence to new life is part of the evidence of life. A crystal does not reproduce itself, it can only be resolved into its elementary constituents; and in the natural course of things, or in an artificial manner, these may be induced to form another crystalline combination. But this is not that continuity of reproduction which links individual to individual, is not procreation wrapped in a cloud of mystery. Herein, it seems, consists a stubborn opposition. Yet, if the distinction between animate and inanimate nature has been recognized as one not entirely absolute; especially if the possibility, nay even the necessity, has been perceived of the primordial generation or parentless origin of the lowest organic beings from inorganic matter (of which more hereafter), and if the nature of nutrition and growth is understood to be entirely dependent on the power of obtaining material, the mystery of reproduction henceforth disappears. Generation is no longer a mystical event; and the origin of an organism in or from an organism, the emission or development of innumerable germs, may, like the origin of a new crystal, be analyzed into the motions. of elements, as yet accessible only to the eye of imagi
nation. By this we mean to say that in the province of reproduction the limits of inquiry are neither narrow nor peculiar. We will therefore now proceed to describe the process of reproduction and development in the animal kingdom.
If, as must be generally admitted, the most essential characteristics are common to the highest and the lowest life, and it is only the complexity of the vital processes, together with the variety of the parts by which they are performed, that give rise to graduated diversities,-it will, of course, be in the simplest organisms that we shall most readily recognize the nature of these vital processes.
The simplest beings, discovered by Haeckel, such as the Protamoba, those minute albuminous masses of sarcode, increase to a certain extent. Why these dimensions should vary only within definite narrow limits, and why, on attaining a certain extent, the molecules should gravitate into two halves, we do not know; at any rate it is an affair of relations of cohesion, theoretically susceptible of computation. It is enough that at a certain size the coherence of the parts is loosened in a central zone, the individual becomes faithless to its name, and divides into two halves, of which each from the moment of separation begins an individual life, while from the commencement of the fission preparations were being made for their self-dependence. This is the simplest case of reproduction, a multiplication by division. Frequently, however, it does not stop at bisection; the motion of the minute constituents, which causes the fission, proceeds in such a manner that the halves are again divided, and the quarters yet again, the whole being thus divided into a greater number of
portions, and the parent-creature is resolved into a swarm of off-shoots.
This multiplication by mere division of the mass presupposes that the organism thus reproducing itself possesses no high complexity. The bisection of a beetle. or a bird is inconceivable as a means of propagation. Yet Stein's valuable observations on the reproductive process of the Infusoria, make us acquainted with organisms standing far above these simple so-called Monera, of which the subdivisions undergo a series of profound metamorphoses, before separating as selfdependent individuals. This transformation, combined with fission, leads to reproduction by gemmation.
As the fission of these low organisms depends on the attainment of a certain limit of growth conditional on adequate nourishment, the case now more frequently occurs that the individual discharges the superfluity of material obtained at a definite part of the body, and forms a bud or gemmule. We are already acquainted with reproduction by gemmation in the simplest organism, the cell; for all healing and cicatrization in higher beings, even to the re-integration of the mutilated limbs. of amphibians, is effected only by the reproduction by fission and gemmation of the elementary morphological constituents. But it lies in the nature of the process of gemmation, that it should extend far higher than fission in the scale of organisms; it is the origination of a new being from one already existing, the latter, meanwhile, preserving its individuality wholly or for the greater part, and yet being able to transfer to the progeny its own characteristics in their full integrity.
The simplest case of gemmation is where the parent
animal produces one or more gemmules similar to itself, capable in their turn of producing similar gemmules. Of this, every collection of corals gives numerous examples, and shows how the diversified appearance of the several genera of coral depends merely on minor modifications of this mode of reproduction. Yet single corals exist in which, on careful comparison, not only may accidental deviations be already discerned, but regularly recurring variations between parent and progeny, as Semper has recently shown in Madrepores and Fungiform corals. This brings us to the highly-important phenomenon of Alternate Generation, which we must elucidate by a few examples before entering upon the nature of sexual reproduction.
Figure 3 shows in A a polype-shaped being with cruciform tentacles, on which its discoverer, Dujardin, bestowed the generic name of Cross-polype, or Stauridium. This animal, growing like a polype upon a stalk, forms above its lower cross, gemmules which make their appearance as spherical balls, gradually assume a bell-like shape, and detach themselves on attaining the structure and form of a Medusa or sea-nettle. The Medusa (termed Cladonema Radiatum, Fig. 3 B) is thus the offspring of its utterly dissimilar parent, the Stauridium; it reproduces itself in the sexual method, and from its eggs proceed Stauridia. The two generations thus alternate; the cross-polype is an intermediate generation in the development of the Medusa, so that the sexual generation never originates directly from its egg.
In the tape-worm, we have an illustration of the same process, only in a somewhat more complicated form. It is known that from the intestinal canal of individuals
afflicted with tape-worm, issue so-called somites or segments of the tape-worm. These somites are usually filled with such an extraordinary number of ova that they seem like mere packets of eggs. It appears, how
ever, from the evolutionary history of the tape-worm, and its relations with other annulosa, namely with leeches and Turbellaria, that notwithstanding their incompleteness and deficiency of organs, these somites are equivalent to sexually mature individuals; or, according to Haeckel's definition, are endowed with personality. If the tape-worm now comported itself like most other animals, somites would be directly developed from its eggs. But to this there is a very circuitous proceeding.