FECUNDATEION. — It is the union of the male germ cell with the female germ cell, that is, of two gametes of different sex. In most organisms, and chiefly in all multicellular organisms, a somatic line is distinguished from a germ line, the latter giving rise to the gametes which, once differentiation has taken place, will be capable of effecting f. The distinction between a somatic line and a germ line is already found in colonial protozoa (Eudormia californica, Volvox) and is the rule among metazoa. There are cases in which the entire individual has the status of a gamete, and in these cases f. is brought about by the union of two entire individuals (hologamy in Chlamydomonads).
Gametes may be grouped into three categories according to their type of differentiation. a) Isogamy: the gametes of the two sexes are morphologically indistinguishable, although physiological differences exist between them (M. Hartmann); b) anisogamy: the gametes of the two sexes are of different size, but there is not yet such a pronounced specific differentiation as in f.; c) oogamy, in which the male gametes are represented by the spermatozoon and the female gametes by the egg.
In metazoa, gametes differentiate in the sex glands, which may be found in separate male and female individuals (gonochorism) or in the same individual (hermaphroditism). The male gonads are called testes and produce spermatozoa; the female gonads are called ovaries and produce eggs. There are cases in which the same organ (ovariotestis) produces spermatozoa and eggs.
The study of isogamy, anisogamy, and oogamy has shown that male and female gametes may undergo different stages of morphological differentiation and that, 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 eggs, on the other hand, differ from one another in their respective morphology and physiology: spermatozoa are very small, in the vast majority of cases motile, and produced in enormous quantities. Motility is provided by a tail or flagellum, which functions as a propulsive organ for the cell in the water or fluids in which the spermatozoon finds itself. Eggs are large cells which may reach considerable dimensions, as, for example, in reptiles and birds; this is due to the accumulation in the egg of materials of a lipoprotein nature, which constitute the vitellus or yolk and accumulate during vitellogenesis. The yolk serves as nourishment for the future embryo until it is able to nourish itself by taking food from the external world. The egg cell is therefore characterized by active synthesis of lipids and proteins, leading to an increase in volume. An exception is constituted by the eggs of placental mammals and of man, in which, given the close relations existing between mother and offspring, the vitellus is very scanty, although the yolk sac is present. Before it is born and suckled by its mother, the new organism is nourished by her through the placenta.
F. may be external or internal. Animals that live in water and release their spermatozoa and eggs into it may be regarded as having external f. (aquatic invertebrates, cyclostomes, fishes, and amphibians). F. is internal not only in some fishes and amphibians that nevertheless live in water, but in all terrestrial animals. In the case of typical internal f., the eggs located in the female’s body are reached by spermatozoa introduced into her genital passages and are fertilized by them. The fertilized eggs may be laid (oviparous animals), or develop within the maternal body (ovoviviparous animals), or establish close relations with the mother through the placenta, a respiratory and nutritive organ (mammals, and, in a less perfect form, certain species of fishes and reptiles). If, after it has been fertilized, the egg is laid on land, it is protected against water loss by a calcareous shell or one of another nature, which breaks when the new organism, having grown at the expense of the yolk, is capable of being independent and self-sufficient.
In every case, whether f. is external or internal, the environment into which the spermatozoa are released is water or aqueous fluids, and in these fluids the spermatozoa move by means of their tails until they encounter the egg. There are many mechanisms that promote the encounter of a spermatozoon with an egg; among these, chemical attractions existing between the gametes and determined by gamones—substances to be discussed below—are of particular importance.
Once emitted, the spermatozoa approach the egg and crowd around it; normally, however, only one spermatozoon enters the egg. There are nevertheless cases in which several spermatozoa penetrate the same egg (polyspermy), but only one takes part in the nuclear phenomena of f., while the others undergo a different fate. When the spermatozoon has penetrated the egg, the latter may already have completed its reduction to half the number of chromosomes and meiosis; in that event, the nucleus of the spermatozoon, or male pronucleus, unites with the nucleus of the egg, or female pronucleus, and the nucleus of the zygote is formed. At the beginning of egg cleavage, the chromosomes, in the diploid number, will be identified from this nucleus (v. CROMOSOMA). The fertilized egg therefore contains paternal genes carried by the spermatozoon and maternal genes present in the egg, and the individual that develops will consequently be a mosaic of paternal and maternal characters. In other cases, the union of the male pronucleus with the female pronucleus is delayed relative to the time of entry of the spermatozoon because the egg has not completed meiosis; once this has occurred, the two pronuclei unite, and the chromosomes are identified, as usual, from the nucleus of the zygote, while embryogenesis begins with cleavage of the egg.
The phenomena of egg maturation and the cytology of f. have been the subject of classic studies (Fol. Amici, O. and P. Hertwig, Strassburger, Boveri, Wilson, Van der Stricht, etc.), and it is not possible here to go beyond what has been said. Certain data concerning the physiology and biochemistry of f. are of considerable interest.
As has already been said, chemical forces exist that promote the union of the spermatozoon with the egg. The first to observe that eggs emitted a substance capable of attracting spermatozoa of the same species was Lillie, who called the substance responsible for the tropism of spermatozoa toward eggs fertilizin. Today, chiefly through the work of Hartmann and his school, as well as that of other authors, it is known that a great number of substances are produced by the egg—gynogamones—and by the spermatozoon—androgamones—which are capable of attracting the germinal elements of the same species. The chemical formula of some of these substances is also known. It is interesting at this point to recall that studies on the action and structure of gamones have been conducted to a large extent on flagellates, that is, on forms in which isogamy sometimes makes it impossible to distinguish morphologically between the gametes of the two sexes, although these produce substances which, if of the same nature, nevertheless have a different action according as they act upon one type of gamete (male) or the other type (female).
As soon as the spermatozoon penetrates the egg, a cortical reaction occurs in it; the cortex of the egg undergoes modification in its submicroscopic and molecular structure, as is also indicated by observations in polarized light (J. Runnstrom, A. Monroy, G. Montalenti). The new orientation of the molecules contributes to the formation of the cortex of the fertilized egg.
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 state whose first clearly visible result is the appearance of the first cleavage furrow. It was therefore thought that the activation of the egg ought to be accompanied by a more intense oxidative metabolism, detectable through more vigorous respiration in the fertilized egg than in the unfertilized egg. Many studies have been conducted on this subject, and the initial results obtained by Warburg on echinoderm eggs were confirmed in lamprey eggs (A. Stefanelli). In amphibian eggs, no difference was found between the unfertilized and fertilized egg, and the same was true of fish eggs (J. Brachet, A. Stefanelli, Zeuthen), while other eggs showed, immediately after f., an actual temporary decrease in oxygen consumption. Whithaker sought an explanation of the problem in the differing size of the eggs and in the standard metabolism of each egg in relation to its own mass, while Stefanelli believes that the as yet unresolved problem may be solved through the ecological study of the various types of eggs.
In conclusion, it is useful to recall that in some cases the egg may develop without the intervention of the «spermatozoon». These are cases of parthenogenesis, or virgin development of the egg, which occur in nature and lead to the formation of perfect individuals in which only the female sex may be represented. Natural parthenogenesis is frequent among organisms lower on the zoological scale, and it can also be induced experimentally in various species of eggs, including those of vertebrates. Experimental parthenogenesis is very often merely an activation of the egg which sometimes leads to the formation of complete embryos or larvae.
Artificial f. must not be confused with parthenogenesis (v. below).
BIBLI.: J. Hammerling and M. Hartmann, Riproduzione e sessualità, Milan 1943; J. Brachet, Endopologie chimique, Paris 1944; A. Monroy, Simporte speciale delle IUBS, Milan 1948; E. Padon, Storia naturale del sesso, Turin 1948; A. Stefanelli, Simporte speciale dell'IUBS, Milan 1948; G. Cotranci, Biologia e zoologia generale, 4th ed., Rome 1949. Enrico Urbani