FECUNDATION. — It is the union of the male germ cell with the female germ cell, that is, of two gametes of different sexes. In the majority of organisms, and especially in all multicellular ones, a distinction is made between a somatic line and a germ line; the latter gives rise to the gametes which, once differentiation has occurred, will be capable of effecting fertilization. The distinction between a somatic line and a germ line is already found in colonial protozoa (e.g., *Endorula californica*, *Volvox*) and is the rule in metazoans. There are cases in which the entire individual has the value of a gamete, and fertilization in these cases is brought about by the union of two entire individuals (gamontogamy in *Chlamydomonas*).
Gametes may be grouped into three categories according to their type of differentiation: a) isogamy: the gametes of the two sexes are morphologically indistinguishable, though there are physiological differences between them (W. Hartmann); b) anisogamy: the gametes of the two sexes differ in size but there is not yet a specific differentiation as marked as in fertilization; c) oogamy, in which the male gametes are represented by the spermatozoon and the female gametes by the ovum.
In metazoans, gametes are differentiated in the sexual glands, which may be found in male and female individuals (gonochorism) or in the same individual (hermaphroditism). The male gametes are produced in the testes and give rise to spermatozoa; the female gonads are the ovaries and produce ova. There are cases in which the same organ (ovotestis) produces both spermatozoa and ova.
The study of isogamy, anisogamy, and oogamy has shown that male and female gametes may exhibit various stages of morphological differentiation. In the simplest forms (flagellates), when there are no morphological differences between the gametes of the two sexes, biochemical differences have in some cases been detected.
Spermatozoa and ova differ from one another in their respective morphology and physiology. Spermatozoa are very small, in the vast majority of cases motile, and produced in very large numbers. Motility is provided by a tail or flagellum which functions as a propulsive organ for the cell in water or in the fluids in which the spermatozoon is transported. Ova are large cells which may reach considerable dimensions, as in reptiles and birds; this is due to the accumulation in the ovum of materials of a lipoproteic nature which constitute the yolk and which accumulate during vitellogenesis. The yolk will serve as nourishment for the future embryo until it is capable of feeding by taking in food from the external environment. The ovum is thus characterized by synthetic activity of lipids and proteins which leads to an increase in volume. An exception is provided by the ova of placental mammals and man, in which, given the close relationship existing between mother and child, the yolk is very scant, though the yolk sac is present. The new being, even before being born and suckled by the mother, is nourished by her through the placenta.
Fertilization may be external or internal. External fertilization may be considered in those animals which, living in water, release spermatozoa and ova into it (aquatic invertebrates, cyclostomes, fishes, and amphibians). Fertilization is internal, apart from some fishes and amphibians which live in water, in all terrestrial animals. In the case of typical internal fertilization, the ova found within the body of the female are reached by spermatozoa introduced into her genital tract and fertilize them. The fertilized ova may be laid (oviparous animals) or develop within the maternal body (ovoviviparous animals), or form close relationships with the mother through the placenta, an organ of respiration and nutrition (mammals, and in a less perfect form in some species of fishes and reptiles). If the ovum, after being fertilized, is laid on land, it is protected against water loss by a calcareous or other kind of shell which breaks when the new organism, which has grown at the expense of the yolk, is capable of being independent and autonomous.
In every case, whether of external or internal fertilization, the environment in which the spermatozoa are released is water or aqueous liquids; in these liquids the spermatozoa move by means of their tails until they encounter the ovum. There are many mechanisms which favour the encounter of a spermatozoon with an ovum, and among these of particular importance are the chemical attractions existing between the gametes, determined by gamones, substances of which more will be said later.
Once emitted, the spermatozoa approach the ovum and crowd around it; normally, however, only one spermatozoon enters the ovum. There are, however, cases in which more than one spermatozoon penetrates the same ovum (polyspermy), but it is always one which takes part in the nuclear phenomena of fertilization, while the others meet with a different fate. When the spermatozoon has penetrated the ovum, it may be that the latter has already completed its reduction to half the number of chromosomes, and then the nucleus of the spermatozoon, or male pronucleus, unites with the nucleus of the ovum, or female pronucleus, and the zygote nucleus is formed, from which, at the beginning of the segmentation of the ovum, the chromosomes will be identified in diploid number (v. CHROMOSOMES). In the fertilized ovum, therefore, paternal genes carried by the spermatozoon and maternal genes present in the ovum are represented, and thus the individual which develops will be a mosaic of paternal and maternal characters. In other cases, the union of the male pronucleus with the female pronucleus is delayed with respect to the time of entry of the spermatozoon because the ovum has not completed meiosis: once this has occurred, the two pronuclei unite and, as usual, chromosomes are identified from the zygote nucleus, and embryogenesis begins with the segmentation of the ovum.
The phenomena of maturation of the ovum and the cytology of fertilization have been the object of classical studies (Fol, Amici, O. P. Hertwig, Strasburger, Boveri, Wilson, Van der Stricht, etc.), and it is not possible to enlarge further upon what has been said here. Of interest are certain data concerning the physiology and biochemistry of fertilization.
As has already been said, there exist forces of a chemical nature which favour the union of the spermatozoon with the ovum. The first to note that ova emitted a substance capable of attracting the spermatozoa of the same species was Lillie, and he called the substance responsible for the tropism of the spermatozoa towards the ova fertilizin. Today, thanks especially to the work of Hartmann and his school, as well as that of other authors, it is known that there exists a large number of substances produced by the ovum, gynogamones, and by the seminal fluid, androgamones, capable of attracting the germinal elements of the same species. The chemical formulae of some of these substances are also known. It is interesting to note at this point that studies on the action and structure of the gamones have been conducted to a large extent on flagellates, that is, on those forms in which sometimes isogamy does not allow the gametes of the two sexes to be distinguished morphologically, though they produce substances which, if of the same nature, have a different action according to whether they act on one type of gamete (male) or the other (female).
As soon as the spermatozoon penetrates the ovum, a cortical reaction occurs in the latter; the cortex of the ovum undergoes changes in its submicroscopic and molecular structure, as is also indicated by observations in polarized light (J. Runnström, A. Monroy, G. Montalenti). The orientation of the molecules in the ovum contributes to the formation of the cortex of the fertilized ovum.
It is certain that the entire metabolism of the egg undergoes modifications following the entry of the spermatozoon; some chemical constituents of the protoplasm are broken down and transformed, and the egg passes from a state of inertia to a dynamic condition whose first clearly visible result is the appearance of the first cleavage furrow. It was therefore thought that the activation of the egg should be accompanied by a more intense oxidative metabolism, detectable by a more intense respiration of the fertilized egg compared to the unfertilized egg. Many studies have been conducted on this subject, and the first results obtained by Warburg on the eggs of echinoderms have been confirmed in the eggs of mollusks (A. Steffen). In the eggs of amphibians no difference has been found between the unfertilized and fertilized egg, nor in fish (J. Brachet, A. Steffenelli, Zeuthen), while other eggs have even shown a monomorphic decrease in oxygen consumption after fertilization. Whitaker sought an explanation for the problem in the different notion of eggs and the standard metabolism of each egg relative to its own mass, and Steffenelli believes that the problem, for now unresolved, may be solved by the ecological study of the various types of eggs.
In conclusion, it is useful to recall that in some cases egg development can occur without the intervention of the spermatozoon. These are cases of parthenogenesis, which develop from the egg in nature and lead to the formation of perfect individuals in which only the female sex may be represented. Natural parthenogenesis is frequent in animals low on the zoological scale and can be experimentally induced in different species of eggs, including those of vertebrates. Experimental parthenogenesis very often is merely an activation of the egg, which sometimes leads to the formation of complete embryos or larvae.
This should not be confused with artificial parthenogenesis (see below).