The Evolution of Man, V.1. - Ernst Haeckel (learn to read activity book TXT) 📗

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).

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.

This inference from the unicellular embryonic form to the unicellular ancestor is so simple, but so important, that we cannot sufficiently emphasise it. We must, therefore, turn next to the question whether there are to-day any unicellular organisms, from the features of which we may draw some approximate conclusion as to the unicellular ancestors of the multicellular organisms. The answer is: Most certainly there are. There are assuredly still unicellular organisms which are, in their whole nature, really nothing more than permanent ova. There are independent unicellular organisms of the simplest character which develop no further, but reproduce themselves as such, without any further growth. We know to-day of a great number of these little beings, such as the gregarinae, flagellata, acineta, infusoria, etc. However, there is one of them that has an especial interest for us, because it at once suggests itself when we raise our question, and it must be regarded as the unicellular being that approaches nearest to the real ancestral form. This organism is the amoeba.

For a long time now we have comprised under the general name of amoebae a number of microscopic unicellular organisms, which are very widely distributed, especially in fresh-water, but also in the ocean; in fact, they have lately been discovered in damp soil. There are also parasitic amoebae which live inside other animals. When we place one of these amoebae in a drop of water under the microscope and examine it with a high power, it generally appears as a roundish particle of a very irregular and varying shape (Figures 1.16 and 1.17). In its soft, slimy, semi-fluid substance, which consists of protoplasm, we see only the solid globular particle it contains, the nucleus. This unicellular body moves about continually, creeping in every direction on the glass on which we are examining it. The movement is effected by the shapeless body thrusting out finger-like processes at various parts of its surface; and these are slowly but continually changing, and drawing the rest of the body after them. After a time, perhaps, the action changes. The amoeba suddenly stands still, withdraws its projections, and assumes a globular shape. In a little while, however, the round body begins to expand again, thrusts out arms in another direction, and moves on once more. These changeable processes are called "false feet," or pseudopodia, because they act physiologically as feet, yet are not special organs in the anatomic sense. They disappear as quickly as they come, and are nothing more than temporary projections of the semi-fluid and structureless body.

(FIGURE 1.17. Division of a unicellular amoeba (Amoeba polypodia) in six stages. (From F.E. Schultze.) the dark spot is the nucleus, the lighter spot a contractile vacuole in the protoplasm. The latter reforms in one of the daughter-cells.)

FIGURE 1.18. Ovum of a sponge (Olynthus). The ovum creeps about in a body of the sponge by thrusting out ever-changing processes. It is indistinguishable from the common amoeba.)

If you touch one of these creeping amoebae with a needle, or put a drop of acid in the water, the whole body at once contracts in consequence of this mechanical or physical stimulus. As a rule, the body then resumes its globular shape. In certain circumstances--for instance, if the impurity of the water lasts some time--the amoeba begins to develop a covering. It exudes a membrane or capsule, which immediately hardens, and assumes the appearance of a round cell with a protective membrane. The amoeba either takes its food directly by imbibition of matter floating in the water, or by pressing into its protoplasmic body solid particles with which it comes in contact. The latter process may be observed at any moment by forcing it to eat. If finely ground colouring matter, such as carmine or indigo, is put into the water, you can see the body of the amoeba pressing these coloured particles into itself, the substance of the cell closing round them. The amoeba can take in food in this way at any point on its surface, without having any special organs for intussusception and digestion, or a real mouth or gut.

The amoeba grows by thus taking in food and dissolving the particles eaten in its protoplasm. When it reaches a certain size by this continual feeding, it begins to reproduce. This is done by the simple process of cleavage (Figure 1.17). First, the nucleus divides into two parts. Then the protoplasm is separated between the two new nuclei, and the whole cell splits into two daughter-cells, the protoplasm gathering about each of the nuclei. The thin bridge of protoplasm which at first connects the daughter-cells soon breaks. Here we have the simple form of direct cleavage of the nuclei. Without mitosis, or formation of threads, the homogeneous nucleus divides into two halves. These move away from each other, and become centres of attraction for the enveloping matter, the protoplasm. The same direct cleavage of the nuclei is also witnessed in the reproduction of many other protists, while other unicellular organisms show the indirect division of the cell.

Hence, although the amoeba is nothing but a simple cell, it is evidently able to accomplish all the functions of the multicellular organism. It moves, feels, nourishes itself, and reproduces. Some kinds of these amoebae can be seen with the naked eye, but most of them are microscopically small. It is for the following reasons that we regard the amoebae as the unicellular organisms which have special phylogenetic (or evolutionary) relations to the ovum. In many of the lower animals the ovum retains its original naked form until fertilisation, develops no membranes, and is then often indistinguishable from the ordinary amoeba. Like the amoebae, these naked ova may thrust out processes, and move about as travelling cells. In the sponges these mobile ova move about freely in the maternal body like independent amoebae (Figure 1.17). They had been observed by earlier scientists, but described as foreign bodies--namely, parasitic amoebae, living parasitically on the body of the sponge. Later, however, it was discovered that they were not parasites, but the ova of the sponge. We also find this remarkable phenomenon among other animals, such as the graceful, bell-shaped zoophytes, which we call polyps and medusae. Their ova remain naked cells, which thrust out amoeboid projections, nourish themselves, and move about. When they have been fertilised, the multicellular organism is formed from them by repeated segmentation.

It is, therefore, no audacious hypothesis, but a perfectly sound conclusion, to regard the amoeba as the particular unicellular organism which offers us an approximate illustration of the ancient common unicellular ancestor of all the metazoa, or multicellular animals. The simple naked amoeba has a less definite and more original character than any other cell. Moreover, there is the fact that recent research has discovered such amoeba-like cells everywhere in the mature body of the multicellular animals. They are found, for instance, in the human blood, side by side with the red corpuscles, as colourless blood-cells; and it is the same with all the vertebrates. They are also found in many of the invertebrates--for instance, in the blood of the snail. I showed, in 1859, that these colourless blood-cells can, like the independent amoebae, take up solid particles, or "eat" (whence they