REPRODUCTION OF ANIMALS A. Asexual. - Many animals possess a more or less limited capacity to repair portions of the body that have been accidentally removed (see Regeneration), and this capacity may be so extensive that, if the whole body be cut in pieces, each portion may grow into a new organism. Such a mode of artificial propagation, familiar in horticultural operations, has been made use of in such animals as sponges, and has been performed experimentally in hydroids and some worms. In many Protozoa asexual reproduction by simple division is a normal event. In Coelentera it is common, the plane of division usually passing through the long axis of the body, as in Actinians and many Hydroids, or being horizontal, as in the repeated divisionsby which medusae are produced from an asexual polyp; the new individual may separate completely, or serve to build up a colonial or compound organism. In some Turbellarians (Microstomum) and Chaetopods (Syllis, Myrianida, Nereis, Eunice viridis (the palolo-worm of Samoa), asexual reproduction occurs in a form that is partly fission and partly budding; portions are constricted transversely or laterally, very much smaller than the whole animal, and these grow out into new animals which may separate or remain attached in chains. In Salps, chains are formed sometimes by transverse constriction, sometimes by budding. True budding is much more common than fission; it occurs in Protozoa, Coelentera, Sponges, Polyzoa, Tunicates and some Flatworms and Chaetopods, the bud being a multicellular portion of the tissues which is partly or completely separated from the parent before it proliferates into the new form. In various larval stages of many animals, asexual reproduction by fission or budding may be produced experimentally or may occur naturally. It has been suggested that cases of identical twins in vertebrates and many monstrous forms, including even dermoid cysts, are due to embryonic asexual fission or budding. The artificial subdivision of young embryos has been performed successfully by several investigators (see Heredity). In Lumbricus trapezoides the gastrula stage of the embryo divides and each half produces a complete individual; and multiplication by budding is common at various stages of the life-history of many parasitic worms. Spore formation, or cellular budding, appears to be limited to the Protozoa amongst animals.
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Apart from the special and probably secondary cases presently to be considered under the subheading parthenogenesis, sexual reproduction or amphimixis may be defined as the production of a new organism from a zygote, and a zygote may be defined as the cell resulting from the conjugation of two gametes or sexual cells derived from the specialized reproductive tissue of the parent or parents. In asexual reproduction by spore formation, the spore proliferates without the aid of another spore; in true sexual reproduction the gametes may be regarded as special kinds of spores which appear in two forms, the eggcell, ovum or female gamete not proceeding to proliferate into a new organism until it has been stimulated by partial or complete fusion with the other form, the spermatozoon or male gamete. The act of fusion or conjugation in question is usually spoken of as fertilization, and the zygote, or starting-point of the new organism, is the fertilized egg-cell. Among protozoa and the lower plants there occur a series of forms of conjugation leading towards the specialized form characteristic of the sexual reproduction of higher animals. The conjugation may be isogamous, that is to say the conjugating cells may be actually or at least apparently indistinguishable. The fusion between the cells may be complete, or may concern only the nuclei. The conjugation may be followed by reproduction, or may apparently have no relation to reproduction. In true sexual reproduction the conjugation is heterogamous, i.e. the gametes are unlike; the fusion is chiefly nuclear, and the process is the prelude of the development of the zygote into the new organism.
In all the Metazoa the gametes arise from special reproductive tissues which are supposed to contain (see Heredity) the reproductive material or germ-plasm. In the lower (or simpler and possibly degenerate Metazoa) the reproductive or germinal tissue consists of a few cells, sometimes in a group, sometimes scattered and sometimes migratory; in the vast majority of the Metazoa the germinal tissue becomes aggregated in distinct organs, of which those that give rise to ova or female gametes are known as the ovaries, and those that give rise to the spermatozoa or male gametes are known as the testes. The ovary and the testis are the primary reproductive organs; the details of their anatomy and position in the various groups need not be discussed here (see Reproductive System).
The male gamete or spermatozoon was first seen in 1677 by Ludwig van Hammen, a pupil of A. Leeuwenhoek, with the microscope that had been constructed by his master. Leeuwenhoek, under the influence of the current preformationist ideas, interpreted these actively moving bodies in the seminal fluids as preformed germs and described them as animalculae spermetia or spermatozoa. Throughout the 18th century the general tendency was to regard them as parasites of no consequence in fertilization. In 1837 R. Wagner established that they were present in all sexually mature males and absent in infertile male hybrids, and in 1841 A. Kblliker showed that they were cells proliferated in the testes. The spermatozoon is one of the smallest of known cells, frequently being no more than one hundred thousandth of the size of the ovum, although the extraordinary case of a small Cypris has been recorded in which the spermatozoa are longer than the animal. It is produced in enormous quantities and relatively to other minute cells is extremely tenacious of life. It may retain its vitality in the male organism for a long time after it has become a separate cell, and may exist for lengthy periods in the female organism. The queen-bee is impregnated only once, and the spermatozoa may remain functional within her body for three years. Lord Avebury (Sir J. Lubbock) has described the case of a female ant which laid fertile eggs thirteen years after she had been impregnated. It is undoubted that in snakes, birds and many mammals, fertilization may not take place for many days after impregnation. The spermatozoa, with a few exceptions, are actively motile, being elongated in shape, with a vibratile tail sometimes provided with a swimming membrane. In a few cases, chiefly of crustaceans, the spermatozoa are spherical with radiating processes, but are capable of amoeboid movements. The cell nucleus is generally situated near the rounded or pointed extremity, with a centrosome immediately behind it, whilst the scanty protoplasm forms the body and vibratile tail; but there appears to be no general significance in the various configurations that occur amongst different animals. The process of spermatogenesis, or production of spermatozoa from the permanent cells of the testis, varies extremely amongst different animals and has been the subject of many elaborate investigations and much confusing nomenclature. Two factors are involved: first, the arrangements to produce a very large crop of cells so to provide for the enormous numbers of spermatozoa produced by most animals; and second, the final changes of shape and of nucleus by which the ripe spermatozoa arise from the indifferent testis-cells, and these processes may to a certain extent overlap. The point of general significance relates to the nuclear changes. The nuclear matter that occurs in the tissue cells of animals, when these cells divide, breaks up into a number of chromosomes constant for each kind of animal, and the final stage of cell division is such that each chromosome splits and contributes a half to each daughter cell, so that the latter come to contain the number of chromosomes peculiar to the animal in which they occur. In the case of spermatozoa, however, a " reducing " division occurs, in which the chromosomes instead of dividing distribute themselves equally between the two daughter cells, with the result that each of the latter contains only half the number peculiar to the species. In its simplest form, what occurs in the last stage of spermatogenesis is that one cell breaks up into four spermatozoa by two successive divisions, the first of which is normal and the second reducing. The nuclear matter of spermatozoa, therefore, contains half the number of chromosomes normal to the tissue cells of the species, and we shall see later that a similar reduction takes place in the formation of the egg. Further complications, however, exist, at least in certain forms. In 1891 H. Henking showed that in a Hemipteran insect of the genus Pyrrochoris, two kinds of spermatozoa are produced in equal numbers, and F. C. Paulmier confirmed the observation in the case of some other insects a few years later, whilst other observers have extended the observation to over a hundred species. In all these cases half the spermatozoa differ from the other half by the presence of what E. B. Wilson calls the " X-element," and which, in the simplest cases, occurs as an unpaired chromosome of the mother cell which passes into one and not the other of the two spermatozoa formed from that mother cell. The matter is still obscure, and it is not certain whether the facts are peculiar to insects or have a parallel in spermatogenesis universally. According to E. B. Wilson, the facts demonstrate that eggs fertilized by spermatozoa with the X-element invariably produce females (see SEx). The female gamete or ovum is in a large number of cases expanded by the presence of food-yolk and protective swathings to form the visible mass known as an egg, and the production of embryos from eggs has been studied from the time of Aristotle and Pliny. Galen had described the human ovaries as testes muliebres, and W. Harvey in 1651 showed that the chick arose from the cicatricula of the yolk of the egg, compared these early stages with corresponding stages in the uterus of mammals, and laid down the general proposition - ovum esse primordium commune omnibus animalibus - that the ovum is a startingpoint common to all animals. In 1664 N. Steno identified the sexual organ of the mammalian female with that of sharks, and first named it the ovary. In 1672 R. De Graaf described the structure of the ovary in birds and mammals, observed the ovum in the oviduct of the rabbit, and repeated Harvey's statement as to the universal occurrence of ova, although he mistook for ova the follicles that now bear his name. In 1825 J. E. Purkyne described the germinal vesicle in the chick, thus distinguishing between the structure of the egg as a whole and the essential germinal area, and in 1827 K. E. von Baer definitely traced the ovum back from the uterus to the oviduct and thence to its origin within the Graafian follicle in the ovary, and thus paved the way for identification of the ovum as a distinct cell arising from the germinal tissue of the ovary. The ovum or female gamete, unlike the spermatozoon, is a large cell, in most cases visible to the naked eye even in the ovary. Also, in definite contrast with the spermatozoon, it is a passive non-motile cell, although in certain cases it is capable of protruding pseudopodia. It is usually spherical, contains a large nucleus, a centrosome and abundant protoplasm, and is generally enclosed in a stout membrane which may or may not have a special aperture known as the micropyle. The protoplasm of all eggs contains nutritive material for the nourishment of the future embryo, and this material may be sufficient in quantity to make the whole cell, although remaining microscopic, conspicuously large, or to expand it to the relatively enormous mass of the yellow yolk of a fowl's egg. Finally, the cellular nature of the ovum is frequently further disguised by its being enclosed in a series of membranes such as the albumen and shell of the fowl's egg. Such complexities are ancillary to the growth or protection of the future embryo, and from the general biological point of view the ovum is to be regarded as a specialized cell derived from the germinal tissue of the ovary, just as the spermatozoon is a specialized cell derived from the corresponding stock of germinal material in the testis. The number of ova produced varies from a very few, as in mammals and birds, to a very large number, as in the herring and many invertebrates, but in all cases the number is relatively small compared with that of the spermatozoa produced by the male of the same species. The details of ovogenesis are more sharply divided than in the case of spermatogenesis into processes connected with the production of a crop of large cells bloated with food-yolk, and the peculiar nuclear changes. The latter changes are generally spoken of as the maturation of the ovum, and in most cases do not begin until the full size has been attained. As in the nuclear changes of spermatogenesis, the details differ in different animals, but the salient feature is that the mature ovum contains, like the ripe spermatozoon, half the number of chromosomes normal to the tissue cells of the animal to which it belongs. The simplest form in which the reduction takes place is that the nucleus of the ovum divides by an ordinary division, each chromosome splitting and sharing itself between the daughter nuclei. Of these nuclei one is extruded from the egg, forming what is called a polar body, and this polar body may again divide by a reducing division, so as to form two polar bodies, each with half the normal number of chromosomes. Finally, the daughter nucleus, remaining in the ovum, also divides by a reducing division, and one of the segments remains to form the nucleus of the ripe ovum, with half the normal number of chromosomes, whilst the other is extruded as a polar body. Very many suggestions as to the meaning of the extrusion of the polar bodies have been made, but the least fanciful of these is to regard the ovum ready for maturation as homologous with the cell about to divide into four spermatozoa; in each case the nucleus divides twice and one of the divisions is a reducing division, so that four daughter nuclei are formed each with half the normal number of chromosomes. Many spermatozoa are required, and each of the four becomes the nucleus of a complete active cell; relatively few ova are required, but each has a large protoplasmic body, and only one of the four becomes a functional mature egg, the other three being simply extruded and so to say wasted. It must be remembered, however, that there is no inherent probability in favour of the apparently simplest explanation of a very complex biological process. It is also to be noted that in many cases the first polar body does not divide, and it is not clearly established that when the first polar body remains single, it is always the result of a; normal nuclear division.
When the mature ova and spermatozoa come together in one of the various ways to be discussed later, fertilization, the conjugation of the gametes to form the zygote, occurs. Alcmaeon (580 B.C.) is believed first to have laid down that fertilization in animals and plants consisted in the material union of the sexual products from both sexes, but it was not until 1761 that it was established experimentally by J. T. Kiilreuter's work on the hybridization of plants. In 1780 L. Spallanzani artificially fertilized the eggs of the frog and tortoise, and successfully introduced seminal fluid into the uterus of the bitch, but came to the erroneous conclusion that it was the fluid medium and not the spermatozoa that caused fertilization. This error was corrected in 1824 by J. L. Prevost and J. B. Dumas, who showed that filtration destroyed the fertilizing power of the fluid. In 1843 M. Barry observed spermatozoa within the egg of the rabbit, whilst in 1849 R. Leuckart observed the fertilization of the frog's egg, and in 1851 H. Nelson noticed the entrance of spermatozoa to the egg of Ascaris, whilst in 1854 a series of observations published independently by T. L. W. Bischoff and Allen Thomson finally and definitely established the fact that ova were fertilized by the actual entrance of spermatozoa. Further advances in microscopical methods enabled a series of observers, of whom the most notable were E. van Beneden, H. Fol and O. Hertwig, to follow and record the details of the process. They made it clear that the chief event in fertilization was entrance into the ovum of the nucleus or head of the spermatozoon where it formed the " male pronucleus," which gradually approached and fused with the female pronucleus or residual nucleus of the ovum. Still later observers, of whom E. B. Wilson is the most conspicuous, have studied the details of the process in many different animals and have shown that the nucleus of the spermatozoon invariably enters the ovum, that the centrosome generally does so, and that the cytoplasm usually plays no part. The nucleus of the zygote or fertilized ovum, then, possesses the number of chromosomes normal in the tissue cells of the animal to which it belongs, but of these half belong to the female gamete and are derived from the germ plasm of the parental ovary, and half to the male gamete or spermatozoon, derived from the germ plasm of the parental testis. The stimulus which leads to and induces the conjugation of the gametes appears to be chemotactic and to consist of some substance positively attractive to the male gamete, liberated by the mature female gamete, but the attraction is mutual, and in the final stages of approach a protoplasmic outgrowth of the ovum towards the spermatozoon frequently occurs. The fertilized zygote proceeds to form the embryo (see Embryology).
Parthenogenesis is the production of the new organism from the female gamete without previous conjugation with the male gamete, and is to be regarded as secondary to and degenerate from true sexual reproduction. Aristotle recognized that it occurred in the bee. In 1745 C. Bonnet showed that it must occur in the case of Aphides or plant-lice, in which throughout the summer there were developed a series of generations consisting entirely of females. R. A. F. de Reaumur repeated the observations, but evaded the difficulty by suggesting that the Aphides were hermaphrodite, an explanation soon afterwards disproved by L. Dufour. In 1849 (Sir) R. Owen brought together the facts as they were" then known and made a remarkable suggestion regarding them. " Not all the progeny of the primary impregnated germ cell are required for the formation of the body in all animals; certain of the derivative germ cells may remain unchanged and become included in that body which has been composed of their metamorphosed and diversely combined or confluent brethren; so included, any derivative germ cell or the nucleus of such may begin and repeat the same processes of growth by imbibition, and of propagation by spontaneous fission, as those to which itself owed its origin." Taking hold of the recently published views of J. J. S. Steenstrup on alternation of generations, he correlated the sexual and asexual alternation in hydroids and so forth with the virgin births of insects and Crustacea, and regarded the one and the other as instances, of the subsequent proliferation of included germ cells, applying the word parthenogenesis to the phenomenon. His theory was a very remarkable anticipation of the germ-plasm theory of A. Weismann, but further knowledge showed that there was an important distinction between the reproduction of the asexual generations described by Steenstrup and the cases of Aphides and Crustacea, the germinal cells in the latter instances being true ova produced from the ovaries of true females, but capable of development without fertilization. In 1856 C. T. E. von Siebold established this fact and limited Owen's term parthenogenesis to the sense in which it is now used, the development without fertilization of ova produced in ovaries. True parthenogenesis occurs frequently amongst Rotifers, and in certain cases (Philodinadae) males either do not exist or are so rare that they have not been discovered. Amongst Crustaceans it is common in Branchiopods and Ostracods; in the case of Daphnids, large thick-shelled ova are produced towards winter, which develop only after fertilization and produce females; the latter, throughout summer, produce thin-shelled ova which do not require fertilization, and from which towards autumn both males and females are produced. Amongst insects it occurs in many forms in many different groups, sometimes occasional, sometimes as a regular occurrence. Apart from Aphides the classical instance is that of the bee, where eggs that are not fertilized develop parthenogenetically and produce only drones. What is known as pathological parthenogenesis has been observed occasionally in higher animals, e.g. the frog, the fowl and certain mammals, whilst in the case of human beings, ovarian cysts in which hair and other structures are produced have been attributed to the incomplete development of parthenogenetic ova. Finally, it has been shown in a number of different instances, notably by J. Loeb, that artificial parthenogenesis may be induced by various mechanical and chemical stimulations. It has been shown that ova may be induced to segment by the presence of spermatozoa belonging even to different classes of the animal kingdom - as, for instance, the ova of echinoderms by the spermatozoa of molluscs. In such cases the resulting embryos have purely maternal characters. A possible interpretation is that spermatozoa have two functions which may be exercised independently; they may act as stimulants to the ovum to segment, and they may convey the paternal qualities. The former function may be replaced by the chemical substances employed in producing artificial parthenogenesis. Juvenile or precocious parthenogenesis, in which there takes place reproduction without fer tilization in immature larvae, has been observed chiefly in insects (Dipterous midges), and to this the term paedogenesis has been applied.
The theory of parthenogenesis remains doubtful. When Weismann and others began to study the polar bodies, they made the remarkable discovery that in some parthenogenetic eggs only one polar body was extruded, but the meaning of this distinction was blurred when other cases were described in which two polar bodies were formed. Later on, Weismann drew attention to the difference between normal and reducing divisions, and it now appears to be clear that, with one set of exceptions, ova which develop without fertilization are those in which no reducing division takes place and which, accordingly, contain the number of chromosomes normal to the tissue cells of the species. Such eggs, in fact, resemble the zygote except that all their chromosomes are of maternal origin and the centrosome which becomes active in the first segmentation is that of the ovum and not, as in normal fertilized eggs, that which came in with the spermatozoon. The case of the bee and other insects in which parthenogenetic development results in the production of males, is doubtful; it appears to be the case that a reduction division has taken place in the maturation of the egg. A. Petrunkevitch has made the ingenious suggestion, that after the reducing division the normal number of chromosomes is restored by the splitting of each into two. Cases of pathological and artificial parthenogenesis would fall into line, on the supposition that the stimulus acted by preventing the occurrence of a reducing division in an ovum otherwise mature. It is to be noticed, however, that such explanations of parthenogenesis are not much more than a formal harmonizing of the behaviour of the chromosomes in the respective cases of fertilized and parthenogenetic development; they do not provide a theory as to why the process occurs.
It has been already stated that the primary organs of reproduction in animals are the germinal tissues producing respectively spermatozoa and ova, and that in most cases these are aggregated to form testes and ovaries. In certain animals there are no accessory organs, and when the reproductive products are ripe, they are discharged directly to the exterior if the gonads are external, as in some Coelentera, or if they are internal, break through into some cavity of the body and escape by rupture of the body-wall or through some natural aperture. In a majority of cases, however, special ducts are developed, which in the male serve primarily for the escape of the spermatozoa, but secondarily may be associated with intromittent organs. Similarly, in the female, the primary function of the gonad ducts is to provide a passage for the ova, but in many cases they serve also for the reception of spermatozoa, for the development of embryos and for the subsequent exit of the young. Associated with the ovary and the oviducts are many kinds of yolk-glands and shell-glands, the function of which is to form nutritive material for the future embryo, to discharge this into or around the ovum, and to provide protective wrappings. Although, in the last resort, fertilization depends on impulses attracting the spermatozoa to the ova, probably chemical in their nature, the necessary proximity is secured in a number of ways. In many simple cases the ripe products are discharged directly into the surrounding water, and impregnation is a matter of accident highly probable because such animals discharge enormous quantities of ova and spermatozoa, are frequently sessile and live in colonies, and are mature about the same time. In other cases, as, for instance, Tunicates and many Molluscs, the spermatozoa are discharged, and, being drawn into the body of the female with the inhalent currents, there fertilize the ova. In yet a number of other cases, there is sexual congress without intromittence. The males of many fish, such as salmon, attend the females about to discharge their ova, and afterwards pour the male fluid over the liberated eggs; whilst amongst other fish the males seek out a suitable locality and prepare some kind of nest to which the female is enticed and which receives first the ova and then the milt. In many other animals, again, as for instance the frog, the male grasps the ripe female, embracing 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 SEx).
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.
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.
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 Longevity. 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 foodsupply.
LITERATURE. - 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. Leeuwenhoek, Epistolae ad societatem regiam Angliam (1719); R. A. F. de Reaumur, Me'moires pour servir d l'histoire des insectes (Paris, 1 7341 74 2); C. Bonnet, euvres d'histoire naturelle et de philosophie (Neuchatel, 1779-1783); L. Spallanzani, Dissertations relative to the Natural History of Animals and Vegetables (Eng. trans., 2nd ed., London, 1789); J. L. Prevost et J. B. Dumas, " Observations relatives a l'appareil generateur des abimaux males," Ann. Sci. Nat. i. (1824); K. E. von Baer, Epistola ad Academiam Scient. Petropolitanam; Heusinger, Zeitschrift, ii. (1828); Leon Dufour, Recherches anatomiques et physiologique sur les Hemipteres (Paris 1833); R. Wagner, " Recherches sur la generation," Ann. Sci. Nat. viii. (1837); A. Kolliker, Uber 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, Beitrdge zur Lehre der Befruchtung (Gottingen 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'oeuf et la fecondation," Arch. de biol. (1883); O. Hertwig, " Das Problem der Befruchtung," Jen. Zeitsch. xviii. (1885). (P. C. M.)
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