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Rh her firmly for a prolonged period, during which ova and spermatozoa are discharged simultaneously. Where internal fertilization occurs, there are usually special accessory organs. In the female, the terminal portion of the gonad-duct, or of the cloaca, is modified to receive the intromittent organ of the male, or to retain and preserve the seminal fluid. In the male, the terminal portion of the gonad-duct may be modified into an intromittent organ or penis, grooved or pierced to serve as a channel by which the semen is passed into the female. In arthropods, ordinary limbs may be modified for this purpose, or special appendages developed; in spiders, the terminal joints of the pedipalps, or second pair of appendages, are enlarged, and are dipped into the semen, which is sometimes shed into a special web, and are used as intromittent organs; in cuttlefish, one of the “arms” is charged with spermatozoa, is inserted into the mantle cavity of the female and there broken off. In many cases there is a temporary apposition of the apertures of the male and female, with an injection from the male without a special intromittent organ. The females are usually passive during coitus, and there are innumerable varieties of clasping organs developed by the male to retain hold of the female. Finally, the various secondary sexual characters which are developed in males and females and induce association between them by appeals to the senses, must be regarded as accessory reproductive organs and processes (see ).

Another set of accessory organs and processes are concerned with what may be termed in the widest sense of the phrase “brood-care.” In many cases the relation between parent and offspring ceases with the extrusion of the fertilized ovum, whilst others display every possible grade of parental care. Many of the lower invertebrates choose special localities in which to deposit the ova or embryos, and glands, the viscid secretion of which serves to bind the ova together or to attach them to some external object, are frequently present. In many insects, elaborate preparations are made; special food-plants are selected, cocoons are woven, or, by means of the special organ known as the ovipositor, the eggs are inserted in the tissues of a living or dead host, or in other cases a supply of food is prepared and stored with the young larvae. The eggs or larvae may be attached to the parent and carried about with it, as in the gills of bivalves, the brood-pouches of the smaller Crustacea, the back of the Surinam toad, the vocal sacs of the frog Rhinoderma, the expanded ends of the oviducts or the marsupial pouch. In a large number of cases the young are nourished directly from the blood of the mother by some kind of placental connexion, as in some of the sharks, in Anablebs, a bony fish, in some lizards and in mammals. In other cases, the young after birth or hatching are fed by the parents, by the special secretion of the mammary glands in the case of mammals, by regurgitated food in many birds and mammals, by salivary secretions or by food obtained and brought to the young by the parents.

Reproductive Period.—In a general way, reproduction begins when the limit of growth has been nearly attained, and the instances of paedogenesis, whether that be parthenogenetic as in midges, or sexual as in the axolotl, must be regarded as an exceptional and special adaptation. In lower animals, where the period of growth is short or indefinite, reproduction begins earlier and is more variable. But, in all cases, surrounding conditions play a great part in hastening or retarding the onset of reproduction. Increased temperature generally accelerates reproductive maturity, excess of food retards it, and sudden privation favours it. In a majority of cases it endures to the end of life, but in some of the higher forms, such as birds and mammals, there is a marked decrease or a cessation of reproductive activity, especially in the case of females, as life advances. In most animals, moreover, periods of reproductive activity alternate with periods of quiescence in a rhythmical series. In its simplest form, the rhythm is seasonal; but although at first associated with actual seasonal changes, it persists in the absence or alteration of these. Many animals brought to Europe from the southern hemisphere come

into reproductive activity at the time of year corresponding to the spring or summer of their native home. “Heat,” menstruation and ovulation in the higher mammals, including man, are rhythmical, and probably physiologically linked, but the ancestral meaning of the periodicity is unknown.

Reproduction and Increase of the Race.—Two distinct factors are involved in this question—the potential fecundity of organisms, and the chances of the young reaching maturity. The first varies with the actual output of zygotes, and is determined partly by the reproductive drain on the individual, and especially the female in cases where the ova are provided with much food-yolk, partly on the duration of reproductive maturity, and partly on the various adaptive and environmental conditions which regulate the chances of the gametes meeting for fertilization. It is to be noted that as the gametes are simply cells proliferating from the germinal tissue, the potential number that can be produced is almost indefinite; and as it is found that in very closely allied forms the actual number produced varies within very wide limits, it may be assumed that potential fecundity is indefinite. The possibility of zygotes reaching maturity varies first with the individuation of the organism concerned—that is to say, the degree of complexity of its structure—and the duration of the period of its growth; and secondly, with the incidence of mortality on the eggs and immature young. It is plain that a parasite capable of living only on a particular host may give rise to myriads of progeny, and yet, from the difficulty of these reaching the only environment in which they can become mature, might not increase more rapidly than an elephant which carries a single foetus for about two years, and guards it for many years after birth. The probable adaptation of the variable reproductive processes to the average conditions of the race is discussed under the heading. It may be added here that the adaptation, in all successful cases, appears to be in excess of what would be required merely to replace the losses caused by death, and that there is ample scope for the Malthusian and Darwinian factors. The rate of reproduction tends to outrun the food supply.

.—Almost any zoological publication may contain matter relating to reproduction, but text-books on Embryology must be specially consulted. The annual volumes of the Zoological Record, under the heading “General Subject” until 1906, and thereafter under “Comprehensive Zoology,” give a classified subject-index of the literature of the year in which references to the separate parts of the subject are given. Amongst the older memoirs referred to in this article the following are the most important: A. Leeuwenboek, Epistolae ad societatem regiam Angliam (1719); R. A. F. de Réaumur, Mémoires pour servir á l'histoire des insectes (Paris, 1734-1742); C. Bonnet, Œuvres d'histoire naturelle et de philosophie (Neuchâtel, 1779-1783); L. Spallanzani, Dissertations relative to the Natural History of Animals and Vegetables (Eng. trans., 2nd ed., London, 1789); J. L. Prévost et J. B. Dumas, “Observations relatives a l'appareil générateur des animaux mâles,” ''Ann. Sci. Nat. i. (1824); K. E. von Baer, Epistola ad Academiam'' ''Scient. Petropolitanam; Heusinger, Zeitschrift'', ii. (1828); Léon Dufour, Recherches anatorniques et physiologigue sur les Hémiptères (Paris, 1833); R. Wagner, “Recherches sur la génération,” ''Ann. Sci.'' Nat. viii. (1837); A. Kölliker, Über das Wesen der sogenannten Saamenthiere, Froriep, Notizen xix. (1841); M. Barry, “Spermatozoa observed within the Mammiferous Ovum,” ''Phil. Trans.'' (1743); J. J. S. Steenstrup, On the Alternation of Generations (Eng. trans., Ray Society, London, 1845); R. Leuckart, Beiträge zur Lehre der Befruchtung (Göttingen Nachrichten, 1849); (Sir) R. Owen, On Parthenogenesis (London, 1849); H. Nelson, “The Reproduction of Ascaris mystax,” ''Phil. Trans. (1852); C. T. E. von Siebold, On a'' True Parthenogenesis in Moths and Bees (Eng. trans., London, 1857); E. van Beneden, “Recherches sur la maturation de l'œuf et la fécondation,” ''Arch. de biol.'' (1883); O. Hertwig, “Das Problem der Befruchtung,” ''Jen. Zeitsch.'' xviii. (1885).
 * (P. C. M.)

The various modes in which plants reproduce their species may be conveniently classified into two groups, namely, vegetative propagation and true reproduction, the distinction between them being roughly this, that whereas in the former the production of the new individual may be effected by the most various parts of the body, in the latter it is always effected by means of a specialised reproductive cell.