No one who has often walked across a sand-beach in summer can have failed to remark what the children call "sand saucers." The name is not a bad one, with the exception that the saucer lacks a bottom; but the form of these circular bands of sand is certainly very like a saucer with the bottom knocked out. Hold one of them against the light and you will see that it is composed of countless transparent spheres, each of the size of a small pin's head. These are the eggs of our common Natica or Sea-Snail. Any one who remembers the outline of this shell will easily understand the process by which its eggs are left lying on the beach in the form I have described. They are laid in the shape of a broad, short ribbon, pressed between the mantle and the shell, and, passing out, cover the outside of the shell, over which they are rolled up, with a kind of glutinous envelope,—for the eggs are held together by a soft glutinous substance. Thus surrounded, the shell, by its natural movements along the beach, soon collects the sand upon it, the particles of which in contact with the glutinous substance of the eggs quickly forms a cement that binds the whole together in a kind of paste. When consolidated, it drops off from the shell, having taken the mould of its form, as it were, and retaining the curve which distinguishes the outline of the Natica. Although these saucers look perfectly round, it will be found that the edges are not soldered together, but are simply lapped one over the other. Every one of the thousand little spheres crowded into such a circle of sand contains an egg. If we follow the development of these eggs, we shall presently find that each one divides into two halves, these again dividing to make four portions, then the four breaking up into eight, and so on, till we may have the yolks divided into no less than sixteen distinct parts. Thus far this process of segmentation is similar to that of the egg in other animals; but, as we shall see hereafter, it seems usually to result only in a change in the quality of its substance, for the portions coalesce again to form one mass, from which a new individual is finally sketched out, at first as a simple embryo, and gradually undergoing all the changes peculiar to its kind, till a new-born animal escapes from the egg. But in the case of the Natica this regular segmentation changes its character, and at a certain period, in a more or less advanced stage of the segmentation, according to the species, each portion of the yolk assumes an individuality of its own, and, instead of uniting again with the rest, begins to subdivide for itself. In our Natica heros, for instance, the common large gray Sea-Snail of our coast, this change takes place when the yolk has subdivided into eight parts. At that time each portion begins a life of its own, not reuniting with its seven twin portions; so that in the end, instead of a single embryo growing out of this yolk, we have eight embryos arising from a single yolk, each one of which undergoes a series of developments similar in all respects to that by which a single embryo is formed from each egg in other animals. We have other Naticas in which the normal number is twelve, others again in which no less than sixteen individuals arise from one yolk. But this process of segmentation, though in these animals it leads to such a multiplication of individuals, is exactly the same as that discovered by K.E. von Baer in the egg of the Frog, and described and figured by Professor Bischof in the egg of the Rabbit, the Dog, the Guinea-Pig, and the Deer, while other embryologists have traced the same process in Birds, Reptiles, and Fishes, as well as in a variety of Articulates, Mollusks, and Radiates.
Multiplication by division occurs also normally in adult animals that have completed their growth. This is especially frequent among Worms; and strange to say, there are species in this Class which never lay eggs before they have already multiplied themselves by self-division.
Another mode of increase is that by budding, as in the Corals and many other Radiates. The most common instance of budding we do not, however, generally associate with this mode of multiplication in the Animal Kingdom, because we are so little accustomed to compare and generalize upon phenomena that we do not see to be directly connected with one another. I allude here to the budding of trees, which year after year enlarge by the addition of new individuals arising from buds. I trust that the usual acceptation of the word individual, used in science simply to designate singleness of existence, will not obscure a correct appreciation of the true relation of buds to their parents and to the beings arising from them. These buds have the same organic significance, whether they drop from the parent stock to become distinct individuals in the common acceptation of the term, or remain connected with the parent stock, as in Corals and in trees, thus forming growing communities of combined individuals. Nor will it matter much in connection with the subject under discussion, whether these buds start from the surface of an animal or sprout in its interior, to be cast off in due time. Neither is the inequality of buds, varying more or less among themselves, any sound reason for overlooking their essential identity of structure. We have seen instances of this among Acalephs, and it is still more apparent among trees which produce simultaneously leaf and flower-buds, and even separate male and female flower-buds, as is the case with our Hazels, Oaks, etc.
It is not, however, my purpose here to describe the various modes of reproduction and multiplication among animals and plants, nor to discuss the merits of the different opinions respecting their numeric increase, according to which some persons hold that all types originated from a few primitive individuals, while others believe that the very numbers now in existence are part of the primitive plan, and essential to the harmonious relations existing between the animal and vegetable world. I would only attempt to show that in the plan of Creation the maintenance of types has been secured through a variety of means, but under such limitations, that, within a narrow range of individual differences, all representatives of one kind of animals agree with one another, whether derived from eggs, or produced by natural division, or by budding; and that the constancy of these normal processes of reproduction, as well as the uniformity of their results, precludes the idea that the specific differences among animals have been produced by the very means that secure their permanence of type. The statement itself implies a contradiction, for it tells us that the same influences prevent and produce change in the condition of the Animal Kingdom. Facts are all against it; there is not a fact known to science by which any single being, in the natural process of reproduction and multiplication, has diverged from the course natural to its kind, or in which a single kind has been transformed into any other. But this once established, and setting aside the idea that Embryology is to explain to us the origin as well as the maintenance of life, it yet has most important lessons for us, and the field it covers is constantly enlarging as the study is pursued. The first and most important result of the science of Embryology was one for which the scientific world was wholly unprepared. Down to our own century, nothing could have been farther from the conception of anatomists and physiologists than the fact now generally admitted, that all animals, without exception, arise from eggs. Though Linnaeus had already expressed this great truth in the sentence so often quoted,—"Omne vivum ex ovo,"—yet he was not himself aware of the significance of his own statement, for the existence of the Mammalian egg was not then dreamed of. Since then the discoveries of von Baer and others have shown not only that the egg is common to all living beings without exception, from the lowest Radiate to the highest Vertebrate, but that its structure is at first identical in all, composed of the same primitive elements and undergoing exactly the same process of growth up to the time when it assumes the special character peculiar to its kind. This is unquestionably one of the most comprehensive generalizations of modern times.
In common parlance, we understand by an egg something of the nature of a hen's egg, a mass of yolk surrounded with white and inclosed in a shell. But to the naturalist, the envelopes of the egg, which vary greatly in different animals, are mere accessories, while the true egg, or, as it is called, the ovarian egg, with which the life of every living being begins, is a minute sphere, uniform in appearance throughout the Animal Kingdom, though its intimate structure is hardly to be reached even with the highest powers of the microscope. Some account of the earlier stages of growth in the egg may not be uninteresting to my readers. I will take the egg of the Turtle as an illustration, since that has been the subject of my own especial study; but, as I do not intend to carry my remarks beyond the period during which the history of all vertebrate eggs is the same, they may be considered of more general application.
It is well known that all organic structures, whether animal or vegetable, are composed of cells. These cells consist of an outside bag inclosing an inner sac, and within that sac there is a dot. The outer bag is filled with semi-transparent fluid, the inner one with a perfectly transparent fluid, while the dot is dark and distinct. In the language of our science, the outer envelope is called the Ectoblast, the inner sac the Mesoblast, and the dot the Entoblast. Although they are peculiarly modified to suit the different organs, these cells never lose this peculiar structure; it may be traced even in the long drawn-out cells of the flesh, which are like mere threads, but yet have their outer and inner sac and their dot,—at least while forming.
In the Turtle the ovary is made up of such cells, spherical at first, but becoming hexagonal under pressure, when they are more closely packed together. Between these ovarian cells the egg originates, and is at first a mere granule, so minute, that, when placed under a very high magnifying power, it is but just visible. This is the incipient egg, and at this stage it differs from the surrounding cells only in being somewhat darker, like a drop of oil, and opaque, instead of transparent and clear like the surrounding cells. Under the microscope it is found to be composed of two substances only: namely, oil and albumen. It increases gradually, and when it has reached a size at which it requires to be magnified one thousand times in order to be distinctly visible, the outside assumes the aspect of a membrane thicker than the interior and forming a coating around it. This is owing not to an addition from outside, but to a change in the consistency of the substance at the surface, which becomes more closely united, more compact, than the loose mass in the centre. Presently we perceive a bright, luminous, transparent spot on the upper side of the egg, near the wall or outer membrane. This is produced by a concentration of the albumen, which now separates from the oil and collects at the upper side of the egg, forming this light spot, called by naturalists the Purkinjean vesicle, after its discoverer, Purkinje. When this albuminous spot becomes somewhat larger, there arises a little dot in the centre,—the germinal dot, as it is called. And now we have a perfect cell-structure, differing from an ordinary cell only in having the inner sac, inclosing the dot, on the side, instead of in the centre. The outer membrane corresponds to the Ectoblast, or outer cell sac, the Purkinjean vesicle to the Mesoblast, or inner cell sac, while the dot in the centre answers to the Entoblast. When the Purkinjean vesicle has completed its growth, it bursts and disappears; but the mass contained in it remains in the same region, and retains the same character, though no longer inclosed as before.
At a later stage of the investigation, we see why the Purkinjean vesicle, or inner sac of the egg, is placed on the side, instead of being at the centre, as in the cell. It arises on that side along which the axis of the little Turtle is to lie,—the opposite side being that corresponding to the lower part of the body. Thus the lighter, more delicate part of the substance of the egg is collected where the upper cavity of the animal, inclosing the nervous system and brain, is to be, while the heavy oily part remains beneath, where the lower cavity, inclosing all the organs of mere material animal existence, is afterwards developed. In other words, when the egg is a mere mass of oil and albumen, not indicating as yet in any way the character of the future animal, and discernible only by the microscope, the distinction is indicated between the brains and the senses, between the organs of instinct and sensation and those of mere animal functions. At that stage of its existence, however, when the egg consists of an outer sac, an inner sac, and a dot, its resemblance to a cell is unmistakable; and, in fact, an egg, when forming, is nothing but a single cell. This comparison is important, because there are both animals and plants which, during their whole existence, consist of a single organic cell, while others are made up of countless millions of such cells. Between these two extremes we have all degrees, from the innumerable cells that build up the body of the highest Vertebrate to the single-celled Worm, and from the myriad cells of the Oak to the single-celled Alga.
But while we recognize the identity of cell-structure and egg-structure at this point in the history of the egg, we must not forget the great distinction between them,—namely, that, while the cells remain component parts of the whole body, the egg separates itself and assumes a distinct individual existence. Even now, while still microscopically small, its individuality begins; other substances collect around it, are absorbed into it, nourish it, serve it. Every being is a centre about which many other things cluster and converge, and which has the power to assimilate to itself the necessary elements of its life. Every egg is already such a centre, differing from the cells that surround it by no material elements, but by the principle of life in which its individuality consists, which is to make it a new being, instead of a fellow-cell with those that build up the body of the parent animal and remain component parts of it. This intangible something is the subtile element that eludes our closest analysis; it is the germ of the immaterial principle according to which the new being is to develop. The physical germ we see; the spiritual germ we cannot see, though we may trace its action on the material elements through which it is expressed.
The first change in the yolk, after the formation of the Purkinjean vesicle, is the appearance of minute dots near the wall at the side opposite the vesicle. These increase in number and size, but remain always on that half of the yolk, leaving the other half of the globe clear. One can hardly conceive the beauty of the egg as seen through the microscope at this period of its growth, when the whole yolk is divided, with the dark granules on one side, while the other side, where the transparent halo of the vesicle is seen, is brilliant with light. With the growth of the egg these granules enlarge, become more distinct, and under the microscope some of them appear to be hollow. They are not round in form, but rather irregular, and under the effect of light they are exceedingly brilliant. Presently, instead of being scattered equally over the space they occupy, they form clusters,—constellations, as it were,—and between these clusters are clear spaces, produced by the separation of the albumen from the oil.