« 이전계속 »
the changes through which the embryo passes on its road from the egg to maturity show a wonderful parallelism to the series of changes through which the organism has passed during the history of its evolution from lower forms.
These results are well worth the labor they have cost, and'they illustrate, more clearly than any other facts in biology, the common nature of all living things. They do not, however, contribute directly to a clearer insight into the laws of heredity.
Here we are still compelled to go beyond the visible phenomena, and to attempt by the scientific use of the imagination to discover the as yet unseen relations which bind them together.
As we enter npon this subject it will be well to bear in mind the wide difference between the end we have in view—the discovery of the secondary laws of heredity— and the attempt to understand its ultimate cause.
The power to reproduce itself, to impress upon dead inorganic matter its own distinctive properties, is one of the fundamental characteristics of living matter; and while we may hope that increase of knowledge may some day enable us to trace the origin of this power, such an attempt forms no part of our present undertaking.
We shall accept without explanation the fact that living matter does thus reproduce itself, and we shall confine ourselves to the attempt to discover why the egg of a star-fish for instance, reproduces a star-fish, and the egg of a bee a bee; to discover the origin of the differences between the various forms of reproduction, rather than the cause of what they have in common.
The phenomena of heredity in the higher animals, as well as the mechanism of ova and male cells through which these phenomena are manifested, have certainly been produced by slow modification, through the influence of conditions which are to a great extent open to study. The attempt to trace their origin and significance is not a pure speculation, bu't a legitimate exercise for the scientific intellect.
As we should expect from the fascinating nature of the subject, there has been no lack of speculation in the past, aud various hypotheses have been proposed from time to. time to account for the phenomena of heredity. These hypotheses differ greatly among themselves, but they may beroughly classed as epigenesis hypotheses, and evolution hypothesis: the word evolution being here used, of course, in its old sense, as contrasted with epigenesis.
The hypothesis of evolution, pure and simple, as advocated by Bonnet and Haller, is that there is contained in the egg or seed or in the male element a perfect but minute organism, aud that the subsequent development of the egg is simply the "evolution" or unfolding of this germ. Up to the end of the last century the prevailing opinion was that each egg contains, in a latent or dormant state, a completely formed organism. The fertilization of the egg was supposed to awaken this dormant germ, to call its latent potential life into activity; and the process of development was regarded as the unfolding and growth of the already fully formed and perfect embryo. The embryo was held to be not produced by, but simply unfolded from the egg, and the act of reproduction was therefore regarded as eduction not production.
According to Huxley (Encyc. Brit., Art. Evolution) "Bonnet affirms that before fecundation the hen's egg contains an excessively minute but complete chick, and that fecundation and incubation simply cause this germ to absorb nutritious matters, which are deposited in the interstices of the elementary structure of which the miniature chick or germ is made up. The consequence of this intussusceptive growth is the "development" or "evolution" of this germ into the visible bird. Thus an organized individual "is a composite body consisting of the original or elementary parts, and of the matters which have been associated with them by the aid of nutrition," so that if these matters could be extracted from the individual, it would, so to speak, become concentrated in a point, and would thus be restored to its primitive condition of a germ "just as by extracting from a bone the calcareous substance which is the source of its hardness it is reduced to its primitive state of gristle and membrane."
"Evolution and development are, for Bonnet, synonymous terms; and since, by evolution he means simply the expansion of that which was invisible into visibility, he was naturally led to the conclusion, at which Leibnitz had arrived by a different line of reasoning, that no such thing as generation exists in nature. The growth of an organism being simply a process of enlargement, as a particle of dry gelatine may swell up by the intussusception 'of water, its death is a shrinkage, such as the melted jelly might undergo on desiccation."
Much more anciently the evolution hypothesis found acceptance in a somewhat different form, and the miniature organism was believed to exist in the male element, and to receive from the egg the nourishment needed for its growth and perfect development.
Loeuwenhoeck's discovery of the motile spermatozoa of animals was regarded as a new basis for this view, and the "sperm-animalcule" was held to be the perfect and living animal ready for unfolding or evolution, the term "spermatozoon," still retained in scientific nomenclature, being a remnant of this old hypothesis. Loeuwenhoeck's discovery inaugurated, in the first half of the last century, the warm dispute between the Animalculists and the Ovists, one side holding that the germ is contained in the egg, and the other that it exists as the seminal animalcule.
It is obvious that, in either form, the evolution theory, as above stated, is logically incomplete, since it only accounts for a single generation. Its advocates were therefore compelled to enlarge it, and to assume that, as each organism thus exists, in a perfect form, in the preceding generation, each germ must contain, on a still smaller scale, the perfect germs of all subsequent generations. Thus Bonnet held, in his hypothesis of emboitement, "that all living things proceed from pre-existing germs, and that these contain, one inclosed within the other, the germs of all future living things; that nothing really new is produced in the living world, but that the germs which develop have existed since the beginning of things." (Huxley, Evolution.)
The advocates of the evolution hypothesis appealed to such facts as the presence of a minute plant inside the acorn, or to the butterfly inside the pupa-skin, in support of their views; but the hypothesis, in its crude state, was quickly overthrown by the first discoveries of modern mioroscopic embryology.
Harvey's studies on the development of the chick, followed by the researches of Wolff, Pander, Von Baer, and the host of embryologists of the last fifty years, show conclusively that the embryo is not unfolded out of, but gradually built up from the egg.
We now know that the eggs of all animals, when they are not complicated by the presence of food, or of peculiar coverings for protection or attachment, are essentially alike in optical structure, and that they are not only like each other, but like the constituent cells of all parts of the body of the organism.
Far from being performed in the egg, we know that the body is built up gradually, step by step, by repeated cell-division, and that the earlier stages of development do not result in the formation of the parts of the perfect body at all, but that they simply give rise to germ-layers, or tracts of cells out of which organs are gradually .formed, and that cells which were at first quite widely separated in the embryo may come at last to enter into the formation of a single organ.
For instance, when the nervous system of a vertebrate first makes its appearance in the embryo, there are no traces of the brain, of the spinal cord, of the nerves or of sense organs. It at first consists of a long group of cells running along the middle line of the body, and presenting no difference from the other cells of its surface. In most cases this elongated group of cells becomes converted into a furrow, and afterwards into a closed tube, the nerve-tube, by the folding together of its edges. The primitive nerve-tube is at first simply a long tube of embryonic cells running along the middle line of the back, and it is a very different thing from the final nervous system of an adult mammal, nor is it in any sense a mammalian nervous system in miniature, for the changes by which it becomes converted into the latter are great and numerous, as well as gradual. Certain parts, such as the eye, are formed only in part from this tract of cells, for the vertebrate eye is the result of the combination of an outgrowth from the embryonic nervous system and an ingrowth from the surface of the head.
The whole history of the nervous system and sense organs is thus seen to directly oppose the view that these organs are present in miniature in the germ.