When we examine a little closer the original features of the ovum, we notice the extremely significant fact that in its first stage the ovum is just the same simple and indefinite structure in the case of man and all the animals (Figure 1.13). We are unable to detect any material difference between them, either in outer shape or internal constitution. Later, though the ova remain unicellular, they differ in size and shape, enclose various kinds of yelk-particles, have different envelopes, and so on. But when we examine them at their birth, in the ovary of the female animal, we find them to be always of the same form in the first stages of their life. In the beginning each ovum is a very simple, roundish, naked, mobile cell, without a membrane; it consists merely of a particle of cytoplasm enclosing a nucleus (Figure 1.13). Special names have been given to these parts of the ovum; the cell-body is called the yelk (vitellus), and the cell-nucleus the germinal vesicle. As a rule, the nucleus of the ovum is soft, and looks like a small pimple or vesicle. Inside it, as in many other cells, there is a nuclear skeleton or frame and a third, hard nuclear body (the nucleolus). In the ovum this is called the germinal spot. Finally, we find in many ova (but not in all) a still further point within the germinal spot, a "nucleolin," which goes by the name of the germinal point. The latter parts (germinal spot and germinal point) have, apparently, a minor importance, in comparison with the other two (the yelk and germinal vesicle). In the yelk we must distinguish the active formative yelk (or protoplasm = first plasm) from the passive nutritive yelk (or deutoplasm = second plasm).
(FIGURE 1.12. Mobile cells from the inflamed eye of a frog (from the watery fluid of the eye, the humor aqueus). The naked cells creep freely about, by (like the amoeba or rhizopods) protruding fine processes from the uncovered protoplasmic body. These bodies vary continually in number, shape, and size. The nucleus of these amoeboid lymph-cells ("travelling cells," or planocytes) is invisible, because concealed by the numbers of fine granules which are scattered in the protoplasm. (From Frey.))
In many of the lower animals (such as sponges, polyps, and medusae) the naked ova retain their original simple appearance until impregnation. But in most animals they at once begin to change; the change consists partly in the formation of connections with the yelk, which serve to nourish the ovum, and partly of external membranes for their protection (the ovolemma, or prochorion). A membrane of this sort is formed in all the mammals in the course of the embryonic process. The little globule is surrounded by a thick capsule of glass-like transparency, the zona pellucida, or ovolemma pellucidum (Figure 1.14). When we examine it closely under the microscope, we see very fine radial streaks in it, piercing the zona, which are really very narrow canals. The human ovum, whether fertilised or not, cannot be distinguished from that of most of the other mammals. It is nearly the same everywhere in form, size, and composition. When it is fully formed, it has a diameter of (on an average) about 1/120 of an inch. When the mammal ovum has been carefully isolated, and held against the light on a glass-plate, it may be seen as a fine point even with the naked eye. The ova of most of the higher mammals are about the same size. The diameter of the ovum is almost always between 1/250 to 1/125 inch. It has always the same globular shape; the same characteristic membrane; the same transparent germinal vesicle with its dark germinal spot. Even when we use the most powerful microscope with its highest power, we can detect no material difference between the ova of man, the ape, the dog, and so on. I do not mean to say that there are no differences between the ova of these different mammals. On the contrary, we are bound to assume that there are such, at least as regards chemical composition. Even the ova of different men must differ from each other; otherwise we should not have a different individual from each ovum. It is true that our crude and imperfect apparatus cannot detect these subtle individual differences, which are probably in the molecular structure. However, such a striking resemblance of their ova in form, so great as to seem to be a complete similarity, is a strong proof of the common parentage of man and the other mammals. From the common germ-form we infer a common stem-form. On the other hand, there are striking peculiarities by which we can easily distinguish the fertilised ovum of the mammal from the fertilised ovum of the birds, amphibia, fishes, and other vertebrates (see the close of Chapter 2.29).
(FIGURE 1.13. Ova of various animals, executing amoeboid movements, highly magnified. All the ova are naked cells of varying shape. In the dark fine-grained protoplasm (yelk) is a large vesicular nucleus (the germinal vesicle), and in this is seen a nuclear body (the germinal spot), in which again we often see a germinal point. Figures A1 to A4 represent the ovum of a sponge (Leuculmis echinus) in four successive movements. B1 to B8 are the ovum of a parasitic crab (Chondracanthus cornutus), in eight successive movements. (From Edward von Beneden.) C1 to C5 show the ovum of the cat in various stages of movement (from Pfluger); Figure P the ovum of a trout; E the ovum of a chicken; F a human ovum.)
The fertilised bird-ovum (Figure 1.15) is notably different. It is true that in its earliest stage (Figure 1.13 E) this ovum also is very like that of the mammal (Figure 1.13 F). But afterwards, while still within the oviduct, it takes up a quantity of nourishment and works this into the familiar large yellow yelk. When we examine a very young ovum in the hen's oviduct, we find it to be a simple, small, naked, amoeboid cell, just like the young ova of other animals (Figure 1.13). But it then grows to the size we are familiar with in the round yelk of the egg. The nucleus of the ovum, or the germinal vesicle, is thus pressed right to the surface of the globular ovum, and is embedded there in a small quantity of transparent matter, the so-called white yelk. This forms a round white spot, which is known as the "tread" (cicatricula) (Figure 1.15 b). From the tread a thin column of the white yelk penetrates through the yellow yelk to the centre of the globular cell, where it swells into a small, central globule (wrongly called the yelk-cavity, or latebra, Figure 1.15 d apostrophe). The yellow yelk-matter which surrounds this white yelk has the appearance in the egg (when boiled hard) of concentric layers (c). The yellow yelk is also enclosed in a delicate structureless membrane (the membrana vitellina, a).
As the large yellow ovum of the bird attains a diameter of several inches in the bigger birds, and encloses round yelk-particles, there was formerly a reluctance to consider it as a simple cell. This was a mistake. Every animal that has only one cell-nucleus, every amoeba, every gregarina, every infusorium, is unicellular, and remains unicellular whatever variety of matter it feeds on. So the ovum remains a simple cell, however much yellow yelk it afterwards accumulates within its protoplasm. It is, of course, different, with the bird's egg when it has been fertilised. The ovum then consists of as many cells as there are nuclei in the tread. Hence, in the fertilised egg which we eat daily, the yellow yelk is already a multicellular body. Its tread is composed of several cells, and is now commonly called the germinal disc. We shall return to this discogastrula in Chapter 1.9.
(FIGURE 1.14. The human ovum, taken from the female ovary, magnified 500 times. The whole ovum is a simple round cell. The chief part of the globular mass is formed by the nuclear yelk (deutoplasm), which is evenly distributed in the active protoplasm, and consists of numbers of fine yelk-granules. In the upper part of the yelk is the transparent round germinal vesicle, which corresponds to the nucleus. This encloses a darker granule, the germinal spot, which shows a nucleolus. The globular yelk is surrounded by the thick transparent germinal membrane (ovolemma, or zona pellucida). This is traversed by numbers of lines as fine as hairs, which are directed radially towards the centre of the ovum. These are called the pore-canals; it is through these that the moving spermatozoa penetrate into the yelk at impregnation.
FIGURE 1.15. A fertilised ovum from the oviduct of a hen. the yellow yelk (c) consists of several concentric layers (d), and is enclosed in a thin yelk-membrane (a). The nucleus or germinal vesicle is seen above in the cicatrix or "tread" (b). From that point the white yelk penetrates to the central yelk-cavity (d apostrophe). The two kinds of yelk do not differ very much.
FIGURE 1.16. A creeping amoeba (highly magnified). The whole organism is a simple naked cell, and moves about by means of the changing arms which it thrusts out of and withdraws into its protoplasmic body. Inside it is the roundish nucleus with its nucleolus.)
When the mature bird-ovum has left the ovary and been fertilised in the oviduct, it covers itself with various membranes which are secreted from the wall of the oviduct. First, the large clear albuminous layer is deposited around the yellow yelk; afterwards, the hard external shell, with a fine inner skin. All these gradually forming envelopes and processes are of no importance in the formation of the embryo; they serve merely for the protection of the original simple ovum. We sometimes find extraordinarily large eggs with strong envelopes in the case of other animals, such as fishes of the shark type. Here, also, the ovum is originally of the same character as it is in the mammal; it is a perfectly simple and naked cell. But, as in the case of the bird, a considerable quantity of nutritive yelk is accumulated inside the original yelk as food for the developing embryo; and various coverings are formed round the egg. The ovum of many other animals has the same internal and external features. They have, however, only a physiological, not a morphological, importance; they have no direct influence on the formation of the foetus. They are partly consumed as food by the embryo, and partly serve as protective envelopes. Hence we may leave them out of consideration altogether here, and restrict ourselves to material points—TO THE SUBSTANTIAL IDENTITY OF THE ORIGINAL OVUM IN MAN AND THE REST OF THE ANIMALS (Figure 1.13).