Fig. 13—Ova of various animals, executing amœboid movements, 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. Figs. A1–A4 represent the ovum of a sponge (Leuculmis echinus) in four successive movements. B1–B8 are the ovum of a parasitic crab (Chondracanthus cornutus), in eight successive movements. (From Edward von Beneden.) C1–C5 show the ovum of the cat in various stages of movement (from Pflüger); Fig. D the ovum of a trout; E the ovum of a chicken; F a human ovum.
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 amœba, 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 Chap. IX.
Fig. 14—The human ovum, taken from the female ovary, magnified. 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.
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 fœtus. 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 (Fig. 13).
Now, let us for the first time make use of our biogenetic law; and directly apply this fundamental law of evolution to the human ovum. We reach a very simple, but very important, conclusion. From the fact that the human ovum and that of all other animals consists of a single cell, it follows immediately, according to the biogenetic law, that all the animals, including man, descend from a unicellular organism. If our biogenetic law is true, if the embryonic development is a summary or condensed recapitulation of the stem-history—and there can be no doubt about it—we are bound to conclude, from the fact that all the ova are at first simple cells, that all the multicellular organisms originally sprang from a unicellular being. And as the original ovum in man and all the other animals has the same simple and indefinite appearance, we may assume with some probability that this unicellular stem-form was the common ancestor of the whole animal world, including man. However, this last hypothesis does not seem to me as inevitable and as absolutely certain as our first conclusion.
Fig. 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′). The two kinds of yelk do not differ very much.