At this period of its growth there is a wonderful resemblance between the appearance of the egg, as seen under the microscope, and the firmament with the celestial bodies. The little clusters or constellations are unequally divided: here and there they are two and two like double stars, or sometimes in threes or fives, or in sevens, recalling the Pleiades, and the clear albuminous tracks between are like the empty spaces separating the stars.
This is no fanciful simile: it is simply true that such is the actual appearance of the yolk at this time; and the idea cannot but suggest itself to the mind, that the thoughts which have been at work in the universe are collected and repeated here within this little egg, which offers us a miniature diagram of the firmament. This is one of the first changes of the yolk, ending by forming regular clusters with a sort of net-work of albumen between, and then this phase of the growth is complete.
Now the clusters of the yolk separate, and next the albumen in its turn concentrates into clusters, and the dark bodies, which have been till now the striking points, give way to the lighter spheres of albumen between which the clusters are scattered. Presently the whole becomes redissolved: these stages of the growth being completed, this little system of worlds is melted, as it were: but while it undergoes this process, the albuminous spheres, after being dissolved, arrange themselves in concentric rings, alternating with rings of granules, around the Purkinjean vesicle. At this time we are again reminded of Saturn and its rings, which seems to have its counterpart here. These rings disappear, and now once more out of the yolk mass loom up little dots as minute as before; but they are round instead of angular, and those nearest the Purkinjean vesicle are smaller and clearer, containing less of oil than the larger and darker ones on the opposite side. From this time the yolk begins to take its color, the oily cells assuming a yellow tint, while the albuminous cells near the vesicle become whiter.
Up to this period the processes in the different cells seem to have been controlled by the different character of the substance of each; but now it would seem that the changes become more independent of physical or material influences, for each kind of cell undergoes the same process. They all assume the ordinary cell character, with outer and inner sac,—the inner sac forming on the side, like the Purkinjean vesicle itself; but it does not retain this position, for, as soon as its wall is formed and it becomes a distinct body, it floats away from the side and takes its place in the centre. Next there arise within it a number of little bodies crystalline in form, and which actually are wax or oil crystals. They increase with great rapidity, the inner sac or mesoblast becoming sometimes so crowded with them, that its shape is affected by the protrusion of their angles. This process goes on till all the cells are so filled by the mesoblast, with its myriad brood of cells, that the outer sac or ectoblast becomes a mere halo around it. Then every mesoblast contracts; the contraction deepens, till it is divided across in both directions, separating thus into four parts, then into eight, then into sixteen, and so on, till every cell is crowded with hundreds of minute mesoblasts, each containing the indication of a central dot or entoblast. At this period every yolk cell is itself like a whole yolk; for each cell is as full of lesser cells as the yolk-bag itself.
When the mesoblast has become thus infinitely subdivided into hundreds of minute spheres, the ectoblast bursts, and the new generations of cells thus set free collect in that part of the egg where the embryonic disk is to arise. This process of segmentation continues to go on downward till the whole yolk is taken in. These myriad cells are in fact the component parts of the little Turtle that is to be. They will undergo certain modifications, to become flesh-cells, blood-cells, brain-cells, and so on, adapting themselves to the different organs they are to build up; but they have as much their definite and appointed share in the formation of the body now as at any later stage of its existence.
We are so accustomed to see life maintained through a variety of complicated organs that we are apt to think this the only way in which it can be manifested; and considering how closely life and the organs through which it is expressed are united, it is natural that we should believe them inseparably connected. But embryological investigations have shown us that in the commencement none of these organs are formed, and yet that the principle of life is active, and that even after they exist, they cannot act, inclosed as they are. In the little Chicken, for instance, before it is hatched, the lungs cannot breathe, for they are surrounded by fluid, the senses are inactive, for they receive no impressions from without, and all those functions establishing its relations with the external world lie dormant, for as yet they are not needed. But they are there, though, as we have seen in the Turtle's egg, they were not there at the beginning. How, then, are they formed? We may answer, that the first function of every organ is to make itself. The building material is, as it were, provided by the process which divides the yolk into innumerable cells, and by the gradual assimilation and modification of this material the organs arise. Before the lungs breathe, they make themselves; before the stomach digests, it makes itself; before the organs of the senses act, they make themselves; before the brain thinks, it makes itself; in a word, before the whole system works, it makes itself; its first office is self-structure.
At the period described above, however, when the new generations of cells are just set free and have taken their place in the region where the new being is to develop, nothing is to be seen of the animal whose life is beginning there, except the filmy disk lying on the surface of the yolk. Next come the layers of white or albumen around the egg, and last the shell which is formed from the lime in the albumen. There is always more or less of lime in albumen, and the hardening of the last layer of white into shell is owing only to the greater proportion of lime in its substance. In the layer next to the shell there is enough of lime to consolidate it slightly, and it forms a membrane; but the white, the membrane, and the shell have all the same quality, except that the proportion of lime is more or less in the different layers.
But, as I have said, the various envelopes of eggs, the presence or absence of a shell, and the absolute size of the egg, are accessory features, belonging not to the egg as egg, but to the special kind of being from which the egg has arisen and into which it is to develop. What is common to all eggs and essential to them all is that which corresponds to the yolk in the bird's egg. But their later mode of development, the degree of perfection acquired by the egg and germ before being laid, the term required for the germ to come to maturity, as well as the frequency and regularity of the broods, are all features varying with the different kinds of animals. There are those that lay eggs once a year at a particular season and then die; so that their existence may be compared to that of annual plants, undergoing their natural growth in a season, to exist during the remainder of the year only in the form of an egg or seed. The majority of Insects belong to this category, as do also our large Jelly-Fishes; many others have a slow growth, extending over several years, during which they reach their maturity, and for a longer or shorter time produce broods at fixed intervals; while others, again, reach their mature state very rapidly, and produce a number of successive generations in a comparatively short time, it may be in a single season.
I do not intend to enter upon the chapter of special differences of development among animals, for in this article I have aimed only to show that the egg lives, that it is itself the young animal, and that the vital principle is active in it from the earliest period of its existence. But I would say to all young students of Embryology that their next aim should be to study those intermediate phases in the life of a young animal, when, having already acquired independent existence, it has not yet reached the condition of the adult. Here lies an inexhaustible mine of valuable information unappropriated, from which, as my limited experience has already taught me, may be gathered the evidence for the solution of the most perplexing problems of our science. Here we shall find the true tests by which to determine the various kinds and different degrees of affinity which animals now living bear not only to one another, but also to those that have preceded them in past times. Here we shall find, not a material connection by which blind laws of matter have evolved the whole creation out of a single germ, but the clue to that intellectual conception which spans the whole series of the geological ages and is perfectly consistent in all its parts. In this sense the present will indeed explain the past, and the young naturalist is happy who enters upon his life of investigation now, when the problems that were dark to all his predecessors have received new light from the sciences of Palaeontology and Embryology.
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