The Evolution of Man, vol 1, Ernst Haeckel [books to read this summer TXT] 📗

tiny transparent ova of the echinoderms are the most convenient for following the details of this important process of impregnation. We can, in this case, easily and successfully accomplish artificial impregnation, and follow the formation of the stem-cell step by step within the space of ten minutes. If we put ripe ova of the star-fish or sea-urchin in a watch glass with sea-water and add a drop of ripe sperm-fluid, we find each ovum impregnated within five minutes. Thousands of the fine, mobile ciliated cells, which we have described as “sperm-threads” (Figure 1.20), make their way to the ova, owing to a sort of chemical sensitive action which may be called “smell.” But only one of these innumerable spermatozoa is chosen—namely, the one that first reaches the ovum by the serpentine motions of its tail, and touches the ovum with its head. At the spot where the point of its head touches the surface of the ovum the protoplasm of the latter is raised in the form of a small wart, the “impregnation rise” (Figure 1.25 A). The spermatozoon then bores its way into this with its head, the tail outside wriggling about all the time (Figure 1.25 B, C). Presently the tail also disappears within the ovum. At the same time the ovum secretes a thin external yelk-membrane (Figure 1.25 C), starting from the point of impregnation; and this prevents any more spermatozoa from entering.

Inside the impregnated ovum we now see a rapid series of most important changes. The pear-shaped head of the sperm-cell, or the “head of the spermatozoon,” grows larger and rounder, and is converted into the male pronucleus (Figure 1.26 s k). This has an attractive influence on the fine granules or particles which are distributed in the protoplasm of the ovum; they arrange themselves in lines in the figure of a star. But the attraction or the “affinity” between the two nuclei is even stronger. They move towards each other inside the yelk with increasing speed, the male (Figure 1.27 s k) going more quickly than the female nucleus (e k). The tiny male nucleus takes with it the radiating mantle which spreads like a star about it. At last the two sexual nuclei touch (usually in the centre of the globular ovum), lie close together, are flattened at the points of contact, and coalesce into a common mass. The small central particle of nuclein which is formed from this combination of the nuclei is the stem-nucleus, or the first segmentation nucleus; the new-formed cell, the product of the impregnation, is our stem-cell, or “first segmentation sphere” (Figure 1.2).

(FIGURE 1.25. Impregnation of the ovum of a star-fish. (From Hertwig.) Only a small part of the surface of the ovum is shown. One of the numerous spermatozoa approaches the “impregnation rise” (A), touches it (B), and then penetrates into the protoplasm of the ovum (C).

FIGURES 1.26 AND 1.27. Impregnation of the ovum of the sea-urchin.

(From Hertwig.) In Figure 1.26 the little sperm-nucleus (sk) moves towards the larger nucleus of the ovum (ek). In Figure 1.27 they nearly touch, and are surrounded by the radiating mantle of protoplasm.)

Hence the one essential point in the process of sexual reproduction or impregnation is the formation of a new cell, the stem-cell, by the combination of two originally different cells, the female ovum and the male spermatozoon. This process is of the highest importance, and merits our closest attention; all that happens in the later development of this first cell and in the life of the organism that comes of it is determined from the first by the chemical and morphological composition of the stem-cell, its nucleus and its body.

We must, therefore, make a very careful study of the rise and structure of the stem-cell.

The first question that arises is as to the two different active elements, the nucleus and the protoplasm, in the actual coalescence.

It is obvious that the nucleus plays the more important part in this.

Hence Hertwig puts his theory of conception in the principle: “Conception consists in the copulation of two cell-nuclei, which come from a male and a female cell.” And as the phenomenon of heredity is inseparably connected with the reproductive process, we may further conclude that these two copulating nuclei “convey the characteristics which are transmitted from parents to offspring.” In this sense I had in 1866 (in the ninth chapter of the General Morphology) ascribed to the reproductive nucleus the function of generation and heredity, and to the nutritive protoplasm the duties of nutrition and adaptation.

As, moreover, there is a complete coalescence of the mutually attracted nuclear substances in conception, and the new nucleus formed (the stem-nucleus) is the real starting-point for the development of the fresh organism, the further conclusion may be drawn that the male nucleus conveys to the child the qualities of the father, and the female nucleus the features of the mother. We must not forget, however, that the protoplasmic bodies of the copulating cells also fuse together in the act of impregnation; the cell-body of the invading spermatozoon (the trunk and tail of the male ciliated cell) is dissolved in the yelk of the female ovum. This coalescence is not so important as that of the nuclei, but it must not be overlooked; and, though this process is not so well known to us, we see clearly at least the formation of the star-like figure (the radial arrangement of the particles in the plasma) in it (Figures 1.26 to 1.27).

The older theories of impregnation generally went astray in regarding the large ovum as the sole base of the new organism, and only ascribed to the spermatozoon the work of stimulating and originating its development. The stimulus which it gave to the ovum was sometimes thought to be purely chemical, at other times rather physical (on the principle of transferred movement), or again a mystic and transcendental process. This error was partly due to the imperfect knowledge at that time of the facts of impregnation, and partly to the striking difference in the sizes of the two sexual cells. Most of the earlier observers thought that the spermatozoon did not penetrate into the ovum. And even when this had been demonstrated, the spermatozoon was believed to disappear in the ovum without leaving a trace.

However, the splendid research made in the last three decades with the finer technical methods of our time has completely exposed the error of this. It has been shown that the tiny sperm-cell is NOT

SUBORDINATED TO, BUT COORDINATED WITH, the large ovum. The nuclei of the two cells, as the vehicles of the hereditary features of the parents, are of equal physiological importance. In some cases we have succeeded in proving that the mass of the active nuclear substance which combines in the copulation of the two sexual nuclei is originally the same for both.

These morphological facts are in perfect harmony with the familiar physiological truth that the child inherits from both parents, and that on the average they are equally distributed. I say “on the average,” because it is well known that a child may have a greater likeness to the father or to the mother; that goes without saying, as far as the primary sexual characters (the sexual glands) are concerned. But it is also possible that the determination of the latter—the weighty determination whether the child is to be a boy or a girl—depends on a slight qualitative or quantitative difference in the nuclein or the coloured nuclear matter which comes from both parents in the act of conception.

The striking differences of the respective sexual cells in size and shape, which occasioned the erroneous views of earlier scientists, are easily explained on the principle of division of labour. The inert, motionless ovum grows in size according to the quantity of provision it stores up in the form of nutritive yelk for the development of the germ. The active swimming sperm-cell is reduced in size in proportion to its need to seek the ovum and bore its way into its yelk. These differences are very conspicuous in the higher animals, but they are much less in the lower animals. In those protists (unicellular plants and animals) which have the first rudiments of sexual reproduction the two copulating cells are at first quite equal. In these cases the act of impregnation is nothing more than a sudden GROWTH, in which the originally simple cell doubles its volume, and is thus prepared for reproduction (cell-division). Afterwards slight differences are seen in the size of the copulating cells; though the smaller ones still have the same shape as the larger ones. It is only when the difference in size is very pronounced that a notable difference in shape is found: the sprightly sperm-cell changes more in shape and the ovum in size.

Quite in harmony with this new conception of the EQUIVALENCE OF THE

TWO GONADS, or the equal physiological importance of the male and female sex-cells and their equal share in the process of heredity, is the important fact established by Hertwig (1875), that in normal impregnation only one single spermatozoon copulates with one ovum; the membrane which is raised on the surface of the yelk immediately after one sperm-cell has penetrated (Figure 1.25 C) prevents any others from entering. All the rivals of the fortunate penetrator are excluded, and die without. But if the ovum passes into a morbid state, if it is made stiff by a lowering of its temperature or stupefied with narcotics (chloroform, morphia, nicotine, etc.), two or more spermatozoa may penetrate into its yelk-body. We then witness polyspermism. The more Hertwig chloroformed the ovum, the more spermatozoa were able to bore their way into its unconscious body.

(FIGURE 1.28. Stem-cell of a rabbit, magnified 200 times. In the centre of the granular protoplasm of the fertilised ovum (d) is seen the little, bright stem-nucleus, z is the ovolemma, with a mucous membrane (h). s are dead spermatozoa.)

These remarkable facts of impregnation are also of the greatest interest in psychology, especially as regards the theory of the cell-soul, which I consider to be its chief foundation. The phenomena we have described can only be understood and explained by ascribing a certain lower degree of psychic activity to the sexual principles.

They FEEL each other’s proximity, and are drawn together by a SENSITIVE impulse (probably related to smell); they MOVE towards each other, and do not rest until they fuse together. Physiologists may say that it is only a question of a peculiar physico-chemical phenomenon, and not a psychic action; but the two cannot be separated. Even the psychic functions, in the strict sense of the word, are only complex physical processes, or “psycho-physical” phenomena, which are determined in all cases exclusively by the chemical composition of their material substratum.

The monistic view of the matter becomes clear enough when we remember the radical importance of impregnation as regards heredity. It is well known that not only the most delicate bodily structures, but also the subtlest traits of mind, are transmitted from the parents to the children. In this the chromatic matter of the male nucleus is just as important a vehicle as the large caryoplasmic substance of the female nucleus; the one transmits the mental features of the father, and the other those of the mother. The blending of the two parental nuclei determines the individual psychic character of the child.

But there is another important psychological question—the most important of all—that has been definitely answered by the recent discoveries in connection with conception. This is the question of the immortality of the soul. No fact throws more light on it and refutes it more convincingly than the elementary process of conception that we have described. For this copulation of the two sexual nuclei (Figures 1.26 and 1.27) indicates the precise moment at which the individual begins to exist. All the bodily and mental features of