divisions (Fig. 3, C, E). The result of the two divisions is the formation of four spermatozoa of equal size and equal capacities, each one of which contains half the number of chromosomes found in the original germ-cell.

In the case of parthenogenetic eggs there is usually only one maturation division. The reason for this is not hard to find.

FIG. 3.-MATURATION OF SPERMATOZOID (AFTER WEISMANN).

(By permission of MR. EDWARD ARNOLD.)

A, Primitive germ-cell, with nucleus. B, spermatocyte. C, the first karyokinesis. D, result of first karyokinesis. E, the second karyokinesis. F, four spermatozoa of equal size, matured (Spz-Spermatozoa).

The parthenogenetic egg has to retain, not only the hereditary substance of the ovocyte, but also the equivalent of the spermatozoon, in view of the fact that it undergoes no fertilisation.

It is time now to say a word concerning the phenomenon of fertilisation, or amphimixis, which plays so important a rôle in the animal world. The very fact that fertilisation, or amphi

OVUM AND SPERMATOZOON

23

mixis, is of such wide occurrence as to be almost synonymous with reproduction itself, is sufficient proof of its importance. Amphimixis consists in the mingling of the two parental elements in reproduction; and as we have seen that the elements necessary to generation are contained in the nuclei of the ovum and spermatozoon respectively, amphimixis is a phenomenon of nuclear blending.

As conducive to amphimixis, we must first of all notice the differentiation of the reproductive cells into male and female. Such differentiation does not seem to affect the fundamental organisation either of the egg or the sperm cell, as is borne out by Boveri's experiment already mentioned. Sexual differentiation would appear to be merely an adaptation which facilitates the mingling of the germinal substance of the two reproductive cells. This mingling is brought about by the chemiotrophic attraction exerted by the egg-cell. The structure of the two cells bears out this view. The spermatozoon is small-immeasurably smaller than the ovum-but it possesses greater facility of movement, while the relative immobility of the ovum is compensated for by the fact that it contains that cytoplasmic substance essential to the maintenance of the life of both cells. And while the ovum furnishes the nutritive matter for both, the spermatozoon, by introducing a new centrosome into the egg, enables the latter to develop by fission; for the centrosome of the ovum, in the majority of species, disappears more or less completely in the course of the egg's maturation. Thus both kinds of germ-cells are essential, and each completes the other, the egg furnishing the nutritive capital, and the spermatozoon the dynamic centre of the karyokinetic process.1

The conception which formerly prevailed concerning amphimixis, and which certain experiments of Maupas on Infusorians seemed to confirm, was that amphimixis afforded a sort of "reviving force" by means of which life was perpetually rejuve

1 Boveri, Das Problem der Befruchtung, pp. 32 f. Jena, 1902.

nated. Maupas, in his experiments, demonstrated that certain Infusorians, when artificially prevented from conjugating, die out. The death of the colony is slow, but sure, and Maupas had no hesitation in ascribing it to senile decay; for, when the artificially imposed conditions were removed, and when conjugation set in again, the degeneration was effectually prevented, and the colony was "rejuvenated." Maupas himself described the phenomenon of conjugation as un rajeunissement karyogamique.

The experiments of Maupas are not, however, as conclusive as their author supposed them to be. Weismann has suggested that it may very likely have been the unnatural conditions under which Maupas maintained his colony during several months, rather than the lack of conjugation, which caused their ultimate decay and death. Weismann has himself experimented with a parthenogenetic species of crustacean, Cypris reptans, and during sixteen years he was able to breed over eighty successive generations without any amphimixis. Weismann rightly remarks that the alleged "rejuvenating" force of some former amphimixis must, in this case, have been an extraordinarily persistent one.1

The truth is that amphimixis, in the case of those species which practise it, is an indispensable condition of development and reproduction. Although asexual reproduction may continue for a certain time, sooner or later amphimixis must again intervene. But it is essentially a phenomenon sui generis. That it is not universally indispensable to reproduction is proved by several instances. A large number of Algæ and Fungi are developed exclusively from asexual spores, and the occurrence of parthenogenetic eggs proves that even differentiated female reproductive cells can, under certain circumstances, develop independently of amphimixis. Certain crustacea reproduce themselves solely by parthenogenesis, and this is the more interesting, since it is known that they formerly exhibited sexual reproduction, for the

1 Weismann, Vorträge über Deszendenztheorie, i. 267.

AMPHIMIXIS AS A CONDITION OF LIFE

25

pouch which used to serve for the reception of the spermatozoa still persists, though it is invariably empty.

In what sense, then, it may be asked, can amphimixis be regarded as a condition of the continuation of life? The reply is that, in the case of those species which reproduce by amphimixis, the conditions of reproduction are such that the germcells of the two sexes are incapable of development by themselves, and that amphimixis, by bringing about a fusion of these elements, permits their subsequent development. We have already noticed how the egg and the sperm-cell complete each other. The egg, in order to be ripe for fertilisation, must pass through certain changes. These include the loss-or, at all events, the degeneration-of the centrosome, and the entrance of the spermatozoon in amphimixis can alone remedy the deficiency and enable the egg to develop. Here, therefore, amphimixis appears as an indispensable condition for the continuance of the life of the species. But even this is not absolutely true. The egg can obviate the apparent necessity of amphimixis by undergoing only a part of the changes involved in maturation. This is well illustrated in artificial parthenogenesis.

The remarkable experiments of Tichomiroff and Loeb have shown that, when certain ova are subjected for a short time to the influence of sulphuric acid or to sea-water with some magnesium chloride added to it, parthenogenetic development may ensue. It seems as if the degeneration of the centrosome of the egg can be thus counteracted. In short, the influence of various kinds of chemical reagents artificially introduced may have the same result as amphimixis, as far as the division of the egg and the development of a larva are concerned.1

1 The observations of Petrunkewitsch have confirmed the supposition that the effect of artificial parthenogenesis in the sea-urchin is to prevent the dissolution of the centrosome of the egg. It is not a new centrosome which is formed, but the original persists, in a sense, "strengthened by the aid of the artificially-introduced salts (vide report in Zoologische Jahrbücher, Supplements band vii., Jena, 1904).

C. THE GERM-PLASM.

Since the great impetus given to scientific research by the work of Darwin, numerous theories have been advanced to facilitate an explanation of the phenomena of inheritance and development. We have had the gemmules of Darwin, the idioplasm of Nägeli, the plastidular perigenesis of Haeckel, the determinants of Weismann, the catagenesis of Cope, the intracellular pangenesis of De Vries. Of all these theories, the one which is most in harmony with the facts, the one which seems most likely to survive-not without modifications, perhaps, but in all its fundamental ideas-is the germ-plasm theory of WeisThe monumental work of Weismann bears the impress of an original and powerful intellect. It is a brilliant construction, consistently thought out, and cleverly adjusted to the facts of which it affords a logical explanation.

mann.

The germ-plasm, according to Weismann, is that substance by means of which the capacity for development and reproduction possessed by an organism is handed on to the offspring. It is the hereditary substance, the vehicle for hereditary transmission, the substance by means of which the continuity of life is secured. Nägeli, before Weismann, had recognised that there are two great kinds of living substance-hereditary substance, or idioplasm; and "nutritive" substance, or trophoplasm; and Nägeli had further expressed the view that the former was

1 Some of the chief works on the subject are those of Darwin, The Descent of Man (1st edition, 1871); Pangenesis, in Nature, iii., No. 78 (1871). H. Spencer, The Principles of Biology, 2 vols. E. Haeckel, Generelle Morphologie, 1866; Die Perigenesis der Plastidule (Berlin, 1876). F. Galton, Hereditary Genius (edition 1892); Pangenesis, in Nature, iv., No. 56 (1871). Nägeli, Mechanisch-physiologische Theorie der Abstammungslehre (1884). P. Geddes and J. A. Thomson, The Evolution of Sex (1889). A. Weismann, Das Keimplasma (1892); Vorträge über Deszendenztheorie, 2 vols. (1902). Delage, L'Hérédité et les grands Problèmes de la Biologie générale (1895).