These cells may perhaps be regarded as collectively making up an undifferentiated ovary, i.e. as cells from which, in another phase, the immature Fluke, the reproductive organs are derived.

As soon as the ovum has attained its definitive size, it very generally, probably universally, gives origin to two polar bodies, or globules, or directive vesicles. The ovular nucleus (germinal vesicle or vesicle of Purkinje with nucleolus or germinal spot) approaches the surface, undergoes karyokinetic changes, and finally one moiety is extruded with a very small amount of protoplasm. After a brief period of rest the phenomenon is repeated. The polar globules may themselves divide again, and the nucleus or nuclear moiety they contain pass through mitotic changes1. In some Rotifera, Crustacea, and Insecta one polar body only is formed, and the ovum then proceeds to segment. Such ova are termed partheno-genetic, and the process parthenogenesis. But in the vast majority of multicellular animals it is necessary for the ovum to be impregnated, i.e. it must fuse or conjugate with another cell, the spermatozoon, just as in some unicellular animals a temporary or permanent conjugation between two individuals is requisite from time to time to perpetuate the race.

The spermatozoon is typically a flagellate cell, with or without the addition of a vibratile membrane; it is rarely amoeboid; sometimes of very various shapes even in the same class (Turbellarid); motile, except in Crustacea and a few other Arthropods2. It is produced by the repeated fission of a cell, or of part of a cell, belonging to a testis, an organ homologous with the ovary. During its evolution, a process analogous to the formation of polar bodies in the ovum, or homologous with it, is supposed to occur very generally3. The testis may co-exist with the ovary in the same animal, which is then said to be monoecious or hermaphrodite; or it may be lodged in another animal of the same species, in that case said to be dioecious or of separate sexes. In hermaphrodite animals the testis may ripen at a different time to the ovary, a phenomenon known as successive hermaphroditism, and in most instances certainly a safeguard against self-impregnation, e. g. in the hermaphrodite Gastropoda. Some hermaphrodites, however, are self-impregnating, such as Cestoda, some Trematoda. The Nematode genus Angiostomum is a unique example of an organism, which though anatomically a female, is yet a self-impregnating hermaphrodite.

The actual process of impregnation is, briefly stated, first the penetration of the spermatozoon into the ovum, either through its envelope at any spot, or by a special aperture, the micropyle, secondly the fusion of the protoplasm of the two cells, which is perhaps an unessential feature, followed thirdly by fusion of the nuclei, often termed the male (spermatozoal) and female (ovular) pronuclei. The two pronuclei approach each other, and the granules of the surrounding protoplasm are arranged round each of them, so as to form a star or aster with a pronucleus as a centre. This aster is most pronounced on the aspects of the pronuclei turned to one another. The ovum has now become an oosperm, and it speedily undergoes fission or segmentation and gastrulation.

1 An egg-shell must be carefully distinguished from structures inclosing a number of ova like the cocoons of the Leech and Earthworm, which are secreted by the surface of the body.

2 An ovary in which every ovarian cell becomes an egg, may be termed panoistic; one in which some only become eggs, others giving origin to secondary yolk;or an egg-membrane, meroistic. The terms are Brandt's, and were originally applied by him to Insectan ovaries.

3 There can be no doubt that a vitellarium is essentially a part of an ovarium. Certain Rhabdocoela prove this point remarkably well; see von Graff, Monographic der Turbellarien, i. Rhabdocoelida, Leipzig, I882, p. I38, on the 'Keimdotterstock.' Granulosa cells, yolk cells, epithelium cells connected with the egg, have very generally a similar origin to the egg itself. See A. Thomson, 'Recent researches on Oogenesis,' Q. J. M. xxvi. I886, p. 602, with lit. given p. 606, and a paper by Korschelt, 'Uber die Entstehung, etc. der versch. Zellenelemente des Insekten-ovariums,' Z. W. Z. xliii. 1886.

1 The formation of polar globules is certainly due to cell-fission; the fact that the nucleus undergoes mitotic changes during their appearance is sufficient to prove the point. They may themselves divide again, and their nuclear fragment show mitosis: see especially Trinchesi, 'Evoluzione nei Molluschi,' Atti Acad. Lyncei, (3), vii. I879, Pls. I and 8. But they are commonly degenerate in structure. For recent observations see A. Thomson, 'Recent Researches on Oogenesis,' Q. J. M. xxvi. 1886, p. 591, with lit. p. 605. Their significance is a difficult question. If a spermatozoon enters the ovum, e. g. in an Asterias before they are formed, no union between the male and female pronuclei takes place until the process is completed. A single polar body is found in partheno-genetic ova (Weismann, SB. Natf. Ges. Freiburg, i. B., iii. (1), 1887), a fact that disproves the view that the bodies are a male element which must be got rid of before impregnation, or a safeguard against self-fertilisation. See the views of Weismann, Nature, xxxvi., p. 607, and Minot, American Naturalist, xiv. 1880, p. 106; cf. Balfour, Comp. Embryology, i. pp. 61-4, and E. van Beneden, 'Recherches sur la fecondation,' Arch. de Biologie, iv. 1883, especially pp. 482, 527, 603 et seqq.

Compare with the account given by the last-named, Carnoy, on the germinal vesicle and polar globules in Ascaris megalocephala, * La Cellule,' Lierre,ii. (1), 1886; in sundry Nematodes, Id. op. cit. iii. 1887, and ibid, in the appendix to the 'conference.' For views connected with the import of the nucleus, etc. to the doctrine of Heredity, see Weismann, 'Die Continuitat des Keimplasma's,' Jena, 1885, summarised by Moseley, Nature, xxxiii. 1885-6, p. 154; Id. 'Die Bedeutung der sexuellen Fortpflanzung,' etc, Jena, 1886, similarly summarised, Nature, xxxiv. 1886, p. 629; also Kolliker, 'Die Bedeutung der Zellkerne,' etc, Z. W. Z. xlii. 1885, summarised in the American Naturalist, xix. 1885, p. 1222; Id. 'Das Karyoplasma,' etc., Z. W. Z. xliv. 1886; O. Hertwig, 'Das Problem der Befruchtung,' etc, J. Z. xviii. 1885, and the chapters on the reproduction of plants in Vines, Physiology of Plants, Cambridge, 1886, or the corresponding lectures in Sachs, 'Physiology of Plants,' transl. by Marshall Ward, Clarendon Press, 1887; also Geddes, 'Theory of Growth,' etc, Proc. Roy. Soc. Edinburgh, 1886.

2 The immobility of the spermatozoa of Crustacea and some other Arthropoda is somewhat doubtful. The spermatozoon of Cypris becomes active when transferred to the female; and that of the Cladoceran Polyphemus shows amoeboid motion (Zaccharias, Z. W. Z. xli. 1885).

3 For the accessory globule of the spermatozoa, see E. van Beneden and Julin, Bull. Acad. Roy. Belg. (3), vii. 1884, p. 322; Brown, Q. J. M. xxv. 1885, pp. 350-1, 357; and A. Thomson, 'Recent Researches on Oogenesis,' Q. J. M. xxvi. 1886, pp. 596-8. The accessory globule has been supposed to get rid of a female element in the developing spermatozoon and thus to be homologous with a polar globule. Such a value can hardly be assigned to the various forms of blastophores, nucleated and non-nucleated, seen in spermatogenesis. The male cell is incapable of further development, i. e. of parthenogenesis in animals where it is highly specialised; so, too, in the higher plants, but in certain of the lower male parthenogenesis appears to occur, see Vines, 'Physiology of Plants,' p. 674; cf. Weismann, 'Continuitat des Keimplasma's,' Jena, 1885, cap. ii. p. 70, on the significance of polar bodies, and cap. iii. p. 88, on the essential character of parthenogenesis; Id. Nature, xxxvi. 1887, p. 607.