Fertilization

“ They stand for the most important phenomenon in the living world. Without it there would be no plants and no animals, excepting a few very low forms of no importance, and of course no human beings.
Fecundation or fertilization is the process of union of the female germ cell with the male germ cell; speaking of animals, it is the process of union of the egg or ovum of the female with the spermatozoön of the male. When a successful union of these two cells takes place a new being is started. The process of fertilization or fecundation is also [83] known as impregnation and conception. ”

“ The influences in the personality of the father and of the mother, which during fertilization find expression in the qualitative constitution of the sperm and of the ovum, respectively, can thus bring about a resultant tendency, acting in one direction or the other with varying force. Thus, after fertilization, we have this resultant tendency toward the formation of a male or female embryo.
But, Düsing continues, at this time the sex is not definitely determined. The influence of the nutritive condition in which the fertilized ovum finds itself has yet to make itself felt. ”

“ Formation of Seed—Fertilization.—In fertilization—the influence in flowering plants of the male-cell in the pollen tube upon the egg-cell in the ovule (see Botany) —there are many circumstances of importance horticulturally, to which, therefore, brief reference must be made. Flowers, generally speaking, are either self-fertilized, cross-fertilized or hybridized. Self-fertilization occurs when the pollen of a given flower affects the egg-cell of the same individual flower. Cross-fertilization varies both in manner and degree. ”

“ The second and most material difference is in the manner of impregnating the eggs of the female. According to the best authorities the fertilization of the eggs of the European variety is accomplished by the passage of the male fluid into the water, and thence between the valves and gills of the female, where the eggs, having been expelled from the ovaries, are lying ready for fertilization. ”

“ The supposition has indeed been made that in the case of parthenogenesis, one fertilization is sufficient for a whole series of generations, but this supposition is not only incapable of proof, but it is contradicted by the fact that certain eggs which may develope parthenogenetically are also capable of fertilization. If, in this case, the power of reproduction were sufficient for development, how is it that the egg is also capable of fertilization; and if the power were insufficient, how is it that the egg can develope parthenogenetically? ”

“ There is no union whatever between the parents for the purpose of fertilization. As soon as a fertilizing cell comes in contact with an ovum it seeks to enter into its substance, and as soon as this has happened, the two cells thus united begin to develop into a very tiny fish. As soon as the change begins, we have the embryo of the fish, which thus corresponds to the embryo of the seed.
There is one great difference between the ovary of the plant and that of the fish. When the plant ovary is ripe, its seeds are shed, and then the ovary itself falls off. ”

“ 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. ”

“ That this mutual exchange and nutrition may take place between the general cells of two bodies is made all the more probable from the experiments already alluded to with regard to chemical fertilization—whereby it has been shown that some ova or egg-cells may be started on a process of subdivision and growth by treatment with certain chemicals, such as weak solutions of strychnine, or common salt, apart from any fertilization by a spermatozoon. ”

“ Remembering the fact that among the higher classes of organisms, fertilization is always effected by combining the sperm-cell of one individual with the germ-cell of another; and joining with it the above fact that among hermaphrodite organisms, the germ-cells developed in any individual are usually not fertilized by sperm-cells developed in the same individual; we see reason for thinking that the essential thing in fertilization, is the union of specially-fitted portions of different organisms. If fertilization depended on the peculiar properties of sperm-cell and germ-cell, as such ”

“ Considering now the period of puberty, we find additional needs, for no boy or girl reaches puberty, under ordinary conditions, without knowing that it brings the possibility of fatherhood and motherhood, brings the possibility of that process that we call fertilization, in which the life of plants and animals begins. The boy or girl who reaches this age has a right to know what fertilization means, and what fertilization implies ”

“ But the developmental processes which lead to the formation of fully developed ova and spermatozoa come to a full stop before fertilization and they do not usually begin again until a spermatozoon has entered an ovum, or until the latter has been stimulated by some other outside means. In some animals and plants eggs may develop regularly without fertilization, the stimulus to development being supplied by ”

“ This view harmonizes with the conclusion, which we saw reason to draw, that fertilization does not depend on any intrinsic peculiarities of sperm-cells and germ-cells, but depends on their derivation from different individuals. It explains the facts that nearly-related individuals are less likely to have offspring than others, and that their offspring, when they have them, are frequently feeble. ”

“ When the pistil has reached a certain stage in growth it becomes ready for fertilization. Pollination having been effected, and the pollen-grain having reached the stigma in angiosperms, or the summit of the nucellus in gymnosperms, it is detained there, and the viscid secretion from the Fertilization. glands of the stigma in the former case, or from the nucellus in the latter, induce the protrusion of the intine as a pollen-tube through the pores of the grain. ”

“ It is true that in plants this stored material is seldom considerable, and that is because the ovum so frequently remains even after fertilization within the living tissues of the plant, and is thence supplied, often very abundantly, with food-stuffs; and, moreover, because the young plant that springs from the fertilized ovum maybe very small and simple, and yet capable of immediately procuring its own nourishment. ”

“ Of course it might be urged that in consequence of selective fertilization only the small eggs are impregnated by a male-producing spermatozoan and the large eggs by a female-producing, and an entirely different interpretation of the facts would be necessitated. ”

“ In other words, there must be selective fertilization, that is, a female ovum must be fertilized by a male sperm, and vice versa. The various assumptions made are not pure speculations, but rest upon many facts. ”

“ Parthenogenesis takes place when the whole of the ancestral germ-plasms, inherited from the parents, are retained in the nucleus of the egg-cell. Development by fertilization makes it necessary that half the number of these ancestral germ-plasms must be first expelled from the egg, the original quantity being again restored by the addition of the sperm-nucleus to the remaining half. ”

“ Fresh systems become formed by a continuation of the process of budding, but the zooids of the secondary systems so [Pg 36] formed are sexual. The ova come to maturity before the spermatozoa, so that cross fertilization takes place. ”

“ The whole subject of heredity is undergoing a thorough revision. What seems to be most needed at the present time is more exact experimentation, carried through several generations, together with more searching investigations into the microscopical constitution of egg and sperm, and close analysis of just what takes place during fertilization and the early stages of the development of the individual. Experiments are being conducted on an extended scale in endowed institutions. ”

“ Most commonly used ARTs rely on fertilization as a first step. This joining of egg and sperm is accompanied by the recombination of the genetic material from the sire and dam, and is often referred to as "shuffling the genetic deck." ”

“ We are, as yet, ignorant for the most part of the causes of this lapse of fertilization. Some light has been thrown on it, however, by recent chemical experiments (the effect of sugar and other water-absorbing solutions) , in which we have succeeded in parthenogenetically developing unfertilized ova.
In the higher animals the complete maturity and development of the specific form are requisite for reproduction, but in many of the lower animals it has been observed recently that ovula and sperm-cells are even formed by the younger specimens in the larva stage. ”

“ When the males are only present in small numbers, it must be very uncertain whether any given female will be fertilized: if therefore the eggs of such a female required fertilization in order to develope, it is clear that there would be great danger of entire failure in this necessary condition. ”

“ But generally the sperm is required to start the egg into activity, and complete and normal development in most organisms probably requires fertilization.
Although most of the stages of sexual evolution are presented in the Volvocaceae, one important stage is there lacking which is especially well illustrated among the brown algae and in the Chaetophoraceae. ”

“ In all higher organisms the sexes are separate, and the reproduction of their kind is a sexual process. The germinal elements involved are seeds and pollen, eggs and sperms. In animals the egg bears all the hereditary qualities from the maternal side, and the sperm those from the paternal side. The intimate mixture of these in fertilization gives great possibilities of variations arising from the different combinations and permutations of the vital units within the germ-plasm. ”

“ After the ovum has reached the womb it remains there for a period of about ten days, after which, if it is not fertilized during that time, it passes out of the womb into the vaginal passage, and so out of the body. But if, at any time after it is ripe for fertilization, that is, from the time it begins its journey from the ovaries to the womb, and while it is in the womb, the ovum is met by the male sperm, it is liable to become fertilized—conception is possible. ”

“ Hence the smaller the chances of any single sperm-cell being successful, the larger is the number of such cells produced, and a direct result of this increase in number is a diminution in size. But why should the sperm-cells remain or become so large in the species in which fertilization is internal? The idea suggests itself that the species in this way gains some advantage, which must be given up in the other cases; although such advantage might consist in assisting the development of the fertilized ovum and not in any increase of the true fertilizing substance. ”

“ But after a time, especially under unfavorable circumstances, a new mode of reproduction appears. Certain cells withdraw from the outer layer into the interior of the colony. Here they are nourished by the other cells and develop into true reproductive elements, eggs and spermatozoa. Fertilization, that is, the union of egg and spermatozoon, or mainly of their nuclei, takes place; and the fertilized egg develops into a new organism. But the other cells, which have been all the time nourishing these, seem now to lack nutriment, strength, or vitality to give rise to a new colony. They die. ”

“ The section of the connectives appear to have no influence on nutrition, but affects reproduction, the attempt at fertilization on the part of the male producing no result, and the impregnated female laying no eggs. ”

“ Similar movements of the feet of the male drive the mass backward and leave the eggs more free for the fertilization that follows. When the eggs have been fertilized and provided with a protecting and aerating mass, the parents break out from the nest and take up their life amongst the trees. ”

“ Hence, no two ova in an ovarium or ovules in a seed-vessel—no two spermatozoa or pollen-cells, can be identical. Whether or not there arise other contrasts, there are certain to arise quantitative contrasts; since the process of nutrition cannot be absolutely alike for all. The reproductive centres must begin to differentiate from the very outset. Such being the necessities of the case, what will happen on any successive or simultaneous fertilizations? Inevitably unlikenesses between the respective parental influences must result. ”

“ It was to save the young embryos from exposure to these and kindred dangers that I, as a volunteer employee of the United [Pg 235] States Fish Commission during the summers of 1897 and 1898, among other things resorted to the artificial fertilization of the eggs in a temporary laboratory, carrying the delicate embryos to the swimming stage and dumping them by thousands into the bay. ”