FERTILIZATION. Before the eggs of most animals and plants can begin their development it is necessary that they should unite with specialized cells derived from the body of a male organism. This union is the essential characteristic of ferti lization. In all higher animals and in all flowering plants the union of the gametes occurs within the body of the mother organism, but in many other instances, e.g., most fishes, ferns, the eggs and spermatozoa, or the latter alone, are shed into water and fertilization occurs in this medium.
In animals, the male cell or spermatozoon is typically small in size and consists of three main parts: (i.) the head, composed almost exclusively of the nucleus, (ii.) the middle piece, (iii.) the tail, which being vibratile, provides the means whereby the cell propels itself rapidly through the medium in which fertilization occurs. All these three parts play an essential role in fertilization. As a rule spermatozoa do not move as long as they lie within the organs of the male's body; they only begin to swim with intense activity when shed into the medium containing the eggs, and even then their active life is limited to a very short period of time, varying from a few minutes in the case of a fish to two days in that of mammals.
The ripe unfertilized egg, on the other hand, is a peculiarly inert cell until after a spermatozoon has rendered it fertile. As it leaves the ovary of oviparous animals, the egg contains all the raw materials and most of the mechanism requisite for the for mation of a complete embryo, and the accumulated yolk from which this embryo is formed often renders the egg a large and conspicuous object.
The process by which a spermatozoon is incorporated into the substance of an egg is most conveniently studied in animals in which fertilization takes place outside the body of the female parent. The simplest and perhaps the best examples are provided by many species of sea-urchins (Echinoidea) which shed their gametes, in large numbers, into the surrounding sea water. If ripe eggs and spermatozoa are mixed together in a drop of sea water it can be seen (under the low powers of the microscope) that the presence of the eggs ex erts a powerful stimulus on the male cells, causing them to swim with greatly increased vigour and rapidity. This effect is due to an exudation from the eggs (ferti lin) which ceases to be produced as soon as the eggs have actually united with spermatozoa. Con tact between egg and spermato zoon is, in animals, not due to mutual attraction but is the re sult of chance collisions between the stationary egg and the motile male cells. In some plants, e.g., ferns, the female system appears to secrete malic, tartaric, or other acid which attracts the spermatozoa towards the eggs.
As soon as a spermatozoon strikes the surface of the egg it begins to burrow rapidly through the gelatinous coat by which the female gamete is surrounded.
In this way many spermatozoa may reach the cortex of a single egg, but only one takes part in the essential act of fertilization. In some cases the unfer tilized egg is surrounded by a tough membranous capsule (as in the salmon) so that the spermatozoa can only reach the egg through a special aperture in the capsule known as the micropyle. In such forms fertilization must occur at the base of the micro pyle, whereas in other types it may usually occur at any point on the egg's surface.
The initial phase of fertilization occurs when the head of the effective spermatozoon becomes attached to the surface of the egg. From this moment onwards the inertia of the egg is lost; it begins to respire actively and the whole cycle of growth and development starts. In many cases a visible change occurs at the egg surface whereby a fertilization membrane is formed, thus enclosing the egg within a spherical container considerably larger than the egg. The actual incorporation of the spermatozoon into the egg is due to the activity of the latter rather than to that of the spermatozoon. At the point of contact beween the cells a small cone-like protrusion forms on the egg surface, and in this fertilization-cone the male cell gradually becomes embedded. The cone then subsides and the spermatozoon is thereby drawn into the egg. The vibratile tail of the sperm, so essential for effecting contact with the egg loses its power of movement soon as the head is attached to the female cell ; subsequent to this it often degenerates or is lost : both sperm head and middle piece enter the substance of the egg.
Shortly after the subsidence of the fertilization cone, the male elements (middle piece and nucleus) rotate about their own axis so that the middle piece instead of being directed towards the peri phery of the egg is now nearer to the centre than is the male nucleus. Both structures move towards the egg nucleus which in turn moves up to meet them. During this movement significant changes occur in both nucleus and middle piece of the sperma tozoon. The former considerably enlarges in size and eventually becomes almost identical in appearance with the egg nucleus. The middle piece of the spermatozoon which is at first a more or less compact structure soon becomes the centre of a series of rays, which are known as the male aster. Some authorities believe that the normal cleavage of an egg cell is made possible by the in corporation into the cell of this male aster. Another aster is believed to be associated with the female nucleus, so that in a fertilized egg the two astral bodies necessary for normal cleavage are both present. By the time the female and male nuclei have come into contact the radiations of the male aster extend through out the whole egg. The nuclei now fuse together to form a zygote nucleus, the radiations from the aster fade away and the process of fertilization is complete. From this point onward the zygote nucleus proceeds to undergo mitotic division and the phenomena of segmentation and development follow in their normal se quence. (See CYTOLOGY and EMBRYOLOGY.) The nucleus of the spermatozoon does not appear to be necessary for the initiation of development, but on the other hand it introduces into all the cells of the developing embryo one complete set of the hereditary factors transmitted by the male parent in its chromosomes.
In the majority of cases an egg can only be fertilized by a spermatozoon if both cells are derived from organisms belonging to the same species. Cases of hybridization are, however, not un common, but in almost every case the offspring of such matings are themselves sterile, e.g., mules, certain hybrid moths, pheas ants. In some cases the failure to obtain hybrid forms is due to an inability of the egg and spermatozoon to unite, whilst in others the foreign spermatozoa having fertilized the eggs give rise to embryos which die at an early stage of development (many bony fishes). An interesting case of natural sterility is provided by some species of hermaphrodite animals (Ascidians). These animals produce both eggs and spermatozoa which, although they mature at the same time, will not unite unless the eggs are derived from one individual and the spermatozoa from another. Similar cases of self-sterility are known among plants.
The reproductive power of an animal depends on three impor tant factors : (I) The number of eggs and spermatozoa which the parents produce, (2) the chances which are provided for an active spermatozoon to reach a fertilizable egg, (3) the chances of a newly fertilized egg completing its development into an adult organism. In many of the lower animals and plants there is no close proximity between the sexes at the moment of liberation of the gametes. Very large numbers of eggs and spermatozoa are shed into the surrounding medium, and fertilization depends solely on chance collisions, although there is often a tendency for a large number of both sexes to shed their gametes at the same moment or in response to a certain set of external conditions, e.g., a particular phase of the tide, or of the moon. A somewhat later phase in the evolution of the fertilization mechanism is reached by those animals, e.g., many fish, in which the male and female individuals come into very close proximity before shed ding their generative cells. Thus a male salmon sheds its sperma tozoa over eggs which have just been deposited in a depression made by the female fish on the gravelly river bed. Even under these conditions, many eggs escape fertilization and better results are obtained by removing ripe fish from the water and "stripping" them of their gametes; these are then mixed together, and the whole mass submerged in water. In all cases of oviparous animals considerable wastage occurs unless the chances of fertilization are high and unless the developing eggs are protected from ex ternal accident and predatory organism. This wastage is largely avoided when fertilization is effected within the body of the female parent, and in such cases the causes of infertility are not easy to analyse. In mammals the most likely causes of infertility appear to be (i.) inability of the spermatozoa to move with suf ficient activity, (ii.) mechanical obstructions which prevent the passage of the sperm into the Fallopian tubes where fertilization occurs, (iii.) absence of fertilizable eggs in the female tract at a time when motile spermatozoa are present, (iv.) inability of the fertilized egg to pass into the uterus and there attach itself normally.
The only vertebrate egg which has yet been induced to develop parthenogenetically is that of the frog. In this case development is initiated by puncturing the surface of the egg with a very fine needle, and no further treatment is required. It is claimed that an aster develops near the puncture. (See PARTHENOGENESIS.) Fertilization in Plants.—In plants the essential union of male and female nuclei is fundamentally the same as in animals. In terrestrial types the male gametes are not capable of autono mous movement but form part of the pollen grains, which are carried to the female system of the flower by wind or other ex ternal agency. Communication between the male generative cell and the female egg is effected by means of the pollen tube which grows from the pollen grain and penetrates into the embryo-sac in which the egg cell is situated. (See ANGIOSPERMS ; FERTILIZA TION.) (J. GR.)