FECUNDATION. - 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 ones, a somatic line is distinguished from a germinal line; the latter gives rise to the gametes which, once differentiation has occurred, will be capable of carrying out fertilization. The distinction of a somatic line from a germinal line is already found in colonial protozoa (Eudorina californica, Volvox) and is the rule in metazoa. There are cases in which the entire individual has the value of a gamete, and fertilization in these cases results from the union of two entire individuals (hologamy in the Chlamydomonads).
Gametes can be grouped into three categories according to their type of differentiation. a) Isogamy: the gametes of the two sexes are morphologically indistinguishable, though physiological differences exist between them (M. Hartmann); b) Anisogamy: the gametes of the two sexes are of different size but there is not yet a specific differentiation as pronounced as in fertilization; c) Oogamy, where the male gametes are represented by the spermatozoon and the female gametes by the egg.
In metazoa the gametes differentiate in the sex glands, which may be found on male and female individuals (gonochorism) or on 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 can have different stages of morphological differentiation, and that in the simplest forms (flagellates), when no morphological differences exist between the gametes of the two sexes, differences of biochemical constitution 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 enormous majority of cases motile, and produced in very great quantities. Motility is provided by a tail or flagellum which functions as the propulsive organ of the cell in the water or in the fluids in which the spermatozoon finds itself. Eggs are large cells which can reach considerable dimensions, as, for example, in reptiles and birds, and this is due to the accumulation in the egg of materials of a lipoprotein nature which constitute the vitellus or yolk and which accumulate during vitellogenesis. The yolk will serve as nourishment for the future embryo until the latter is able to feed itself by taking food from the external world. The egg cell is therefore characterized by an active synthesis of lipids and proteins which leads it to an increase in volume. The eggs of placental mammals and of man are exceptions, since in them, given the close relations existing between mother and child, the yolk is very scanty although 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 can be external or internal. Animals that live in water and there release spermatozoa and eggs (aquatic invertebrates, cyclostomes, fish and amphibians) may be considered as having external fertilization. Fertilization is internal, besides in some fish and amphibians that nevertheless live in water, in all terrestrial animals. In the case of typical internal fertilization, the eggs that are found in the body of the female are reached by the spermatozoa introduced into her genital tracts, and these fertilize them. The fertilized eggs can be laid (oviparous animals) or develop in the maternal body (ovoviviparous animals) or enter into close relations with the mother through the placenta, an organ of respiration and nutrition (mammals, and in a less perfect form in some species of fish and reptiles). If the egg, after it has been fertilized, 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, which has grown at the expense of the yolk, is able to be independent and autonomous.
In every case, whether of external or of internal fertilization, the environment into which the spermatozoa are released is water or aqueous fluids, and in these fluids the spermatozoa move by the movements of the tail until they encounter the egg. There exist many mechanisms that favor the meeting of a spermatozoon with an egg, and among these of particular importance are the attractions of a chemical character existing between the gametes, determined by the gamones, substances of which we shall speak later.
Once emitted, the spermatozoa approach the egg and crowd around it; as a rule, however, only one spermatozoon enters the egg. There are, however, cases in which several spermatozoa penetrate the same egg (polyspermy), but it is always one that takes part in the nuclear phenomena of fertilization, while the others undergo a different fate. When the spermatozoon has penetrated the egg, it may be that the latter has already completed its reduction to half the number of chromosomes and its meiosis, and then 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 constituted, from which, at the beginning of the segmentation of the egg, the chromosomes in diploid number will be identified (v. cromosomi). In the fertilized egg there are therefore represented paternal genes carried by the spermatozoon and maternal genes present in the egg, and consequently the individual that will develop will 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 from the nucleus of the zygote the chromosomes are identified, as is normal, and with the segmentation of the egg embryogenesis begins.
The phenomena of maturation of the egg and the cytology of fertilization have been the subject of classical studies (Fol, Amici, O. and P. Hertwig, Strasburger, Boveri, Wilson, Van der Stricht, etc.) and it is not possible here to go beyond what has been said. Some data concerning the physiology and biochemistry of fertilization are rich in interest.
As has already been said, there exist forces of a chemical nature that favor the union of the spermatozoon with the egg. The first to notice that eggs emitted a substance capable of attracting the spermatozoa of the same species was Lillie, and he 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 of other authors, it is known that there exists a great number of substances produced by the egg, gynogamones, and by the spermatozoon, androgamones, capable of attracting the germinal elements of the same species. For some of these substances the chemical formula is also known. It is interesting at this point to recall that the studies on the action and on the 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, but these produce substances which, if they are of the same nature, nevertheless have a different action according to whether they act on one type of gamete (male) or on the other type (female).
As soon as the spermatozoon penetrates the egg, a cortical reaction is produced in the latter; the cortex of the egg changes 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; certain chemical constituents of the protoplasm are broken down and transformed, and the egg passes from a condition of inertia to a dynamic condition whose first clearly visible result is the appearance of the first cleavage furrow. It had therefore been thought that the activation of the egg must be accompanied by a more intense oxidative metabolism, detectable as a more intense respiration of the fertilized egg compared with the virgin egg. Many studies have been conducted on this subject, and the first results obtained by Warburg on echinoderm eggs were confirmed in the eggs of lampreys (A. Stefanelli). In amphibian eggs no difference was found between the virgin egg and the fertilized egg, and likewise in fish (J. Brachet, A. Stefanelli, Zeuthen), while other eggs even showed, after fertilization, a momentary decrease in oxygen consumption. Whithaker sought an explanation of the problem in the different size of the eggs and in the standard metabolism of each egg relative to its own mass, and Stefanelli thinks that the problem, so far unsolved, may be resolved through the ecological study of the various types of eggs.
In conclusion to what has been said, it is useful to recall that in some cases the development of the egg can occur without the intervention of the spermatozoon. These are the cases of parthenogenesis, or virgin development of the egg, which are encountered in nature and lead to the formation of perfect individuals in which only the female sex may be represented. Natural parthenogenesis is frequent in organisms low on the zoological scale and can 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.
Not to be confused with parthenogenesis is artificial fertilization (v. below).
Bibliography6 works
Enrico Urbani