Sperm cell

“ The process of fertilization is now known in all its details in a great number of animals in the most diverse groups; it is everywhere the same in its essential features; there is always only one sperm-cell which normally enters into conjugation with the ovum-nucleus, and in every case the sperm-cell, however minute it may be to begin with, forms a nucleus nearly or exactly as large as the nucleus of the ovum, and in all cases it contains the same number of chromosomes as the nucleus of the ovum. ”

“ When it reaches the ovary, the sperm cell penetrates an ovule by making its way through a little hole called the micropyle. It then grows toward a clear bit of protoplasm known as the embryo sac. The embryo sac is an ovoid space, microscopic in size, filled with semifluid protoplasm containing several nuclei. (See Figure.) One of the nuclei, with the protoplasm immediately surrounding it, is called the egg cell. It is this cell that the sperm nucleus of the pollen tube grows toward; ultimately the sperm nucleus reaches the egg nucleus and unites with it. ”

“ 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 ”

“ Now, in the case of reproduction by budding, as in the hydra, a new individual is produced from some group of cells in the parent organism. From this it is but a step—a step, however, of the utmost importance—to the production of a new individual from a single cell from the tissues of the parental organism. Such a reproductive cell is called an egg-cell, or ovum. In the great majority of cases, to enable the ovum to develop into a new individual, it is necessary that the egg-cell should conjugate or fuse with a minute, active sperm-cell, generally derived from a different parent. ”

“ The human spermatozoon measures about one six-hundredth of an inch in length. It is composed of protoplasm, the substance of which all living creatures are composed. The spermatozoa are believed to be developed from a parent sperm-cell, by the process of segmentation or subdivision, which process is common to all cell-life. The numerous spermatozoa dwell in a gelatinous substance, which, mingling with the other fluidic secretions of the glands of the male, constitutes the male seminal fluid, sperm, or semen, which is ejaculated by the male during the process of copulation. ”

“ The central cell, which contains dense, glistening protoplasm, is destitute of chlorophyll, but the outer cells have a few small chloroplasts. The former divides repeatedly, until a mass of about thirty-two sperm cells is formed, each giving rise to a large spirally-coiled spermatozoid. When ripe, the mass of sperm cells crowds so upon the outer cells as to render them almost invisible, and as they ripen they separate by a partial dissolving of the division walls. ”

“ Each contains a single ovum, or egg-cell, surrounded by a capsule. It, too, and the cells which surround it would appear to be developed from the interstitial cells. It grows rapidly at the expense of the surrounding tissue, but when mature, it bursts through the enveloping capsule, and is freely exposed. A sperm-cell, which seems, in some cases at least, to be produced by the same individual, now unites with it; the egg-cell then begins to undergo division, becomes detached, falls to the bottom, and develops into a young hydra. ”

“ In higher orders of animals, fecundation occurs internally, the conjunction of the sperm and germ cells requiring the conjugation of the male and female sexual organs. The sperm-cells of the male furnish the quickening principle, which sets in play all the generative energies, while the germ-cell, susceptible to its vivifying presence, responds with all the conditions necessary to evolution. The special laboratory which furnishes spermatic material is the testes, while the stroma of the ovaries contributes the germ-cell. ”

“ The sperm cells and egg cells are poured out into the water by the adults, and the sperm cell, which, like nearly all sperm cells, has a vibratory, taillike flagellum to propel it, reaches and fertilizes the egg cell. ”

“ But, reduced as it is to little more than a bare nucleus without any envelope of nutritive matter, the sperm cell cannot support the intense vital activity called for in the initial stages of the life of a new being, and therefore sperm cells, like the germ cells, though for a different reason, would be incapable of mutual conjugation, even if the element of mutual attraction existed among them. ”

“ In non-sexual modes one individual becomes two; in this, two individual cells unite to form one. It is an expensive, even wasteful mode unless attended with some great advantage. The nature of this advantage we will presently see.
Thus far we have given only the lowest form of sexual generation. The two sexual elements only, germ-cell and sperm-cell are separated from each other, but not yet even the sexual organs, ovary and spermary, much less the sexual individuals, male and female. ”

“ Usually the two daughter nuclei are formed in an undivided cell. The resting-stage between the two divisions is only partial. The nucleolus appears and divides into two (figs. 33-36) , and the chromosomes change into the dyad form (fig. 36) , in which they come into the second maturation spindle (figs. 37, 38) . The equatorial plate again shows 26 chromosomes (fig. 39) . The formation of the spermatozoa is peculiar in that the original spermatocyte cell-body, as a rule, does not divide; but the four nuclei resulting from the two maturation divisions develop into sperm-heads in one cell. ”

“ The nucleus of the sperm and of the egg unite in a very complex and precise manner. In other organisms, and probably also in human beings, the entry of a single sperm to the egg cell shuts out the possibility of other sperms fusing with them, because directly it has been fertilized, the egg cell exudes a film of substance which antagonizes the other sperms, and which ultimately forms a filmy skin around itself. ”

“ Corroborative evidence appears in the contrast which persists in all the higher forms between the relatively large female-cell or germ and the microscopical male-cell or sperm, as also in the absorption of the male cellule by the female cellule. In the sexual cells there is no character in which differentiation goes so far as that of size. [24] The female cell is always much larger than the male; where the former is swollen with the reserve food, the spermatozoa may be less than a millionth of its volume. In the human species an ovum is about 3000 times as large as spermatozoa. ”

“ It should be realised that all the proper, medical methods of controlling pregnancy consist, not in destroying an already growing embryo, but in preventing the male sperm from reaching the unfertilised egg cell. This may be done either by shutting the sperms away from the opening of the womb, or by securing the death of all (instead of the natural death of all but one) of the two to six hundred million sperms which enter the woman. ”

“ The single sperm which unites with it is a still more minute fleck, and is little more than a nucleus with a film of protoplasm round it, and a long cilium or hair-like continuation which it lashes to and fro, and thus propels itself or swims towards the egg cell. ”

“ The germ cells of the woman, though immeasurably less numerous than the male germ cells (the spermatozoa) , yet develop uselessly over and over again in every celibate as well as in every married woman. And myriads of sperm cells are destroyed even in the process of the act which does ensure fertilization of the woman by the single favored spermatozoön. If the theologians really mean what they say, and demand the voluntary effort of complete celibacy from all men, save for the purpose of procreation, this will not achieve their end of preventing the wastage of all potential life ”

“ Even now no adequate study of the subject has been made, but it appears that whilst a female mule can be, and sometimes is, successfully mated to a horse or an ass, giving birth to a foal, the male mule does not produce fully-formed spermatozoa. What precisely is the nature of this failure, what the ultimate microscopic condition of the sperm cells in infertile male mules, or in any other infertile male hybrids, has not yet been properly worked out by modern cytological methods. ”

“ Let us think of the enormous number of sperm-cells which are produced by many animals, and indeed by the highest of them—an almost incalculably large number which certainly goes far beyond millions. Let us assume that in Man there may be 12,870 million spermatozoa, then, with sixteen ids, and with an equally frequent occurrence of all possible combinations of germ-plasm—there would be 12,870—there would be a million of each type containing identical germ-plasm. The danger that several ova would be fertilized by identical sperm-cells would be by no means small. ”

“ The individual germ-cells which, in {330}succession or simultaneously, are separated from the same parent, can never be exactly alike; nor can the sperm-cells which fertilize them. When treating of the instability of the homogeneous (First Principles, § 149) , we saw that no two parts of any aggregate can be similarly conditioned with respect to incident forces; and that being subject to forces that are more or less unlike, they must become more or less unlike. Hence, no two ova in an ovarium or ovules in a seed-vessel—no two spermatozoa or pollen-cells, can be identical. ”

“ The spermatozoon is one of the smallest of known cells, frequently being no more than one hundred thousandth of the size of the ovum, although the extraordinary case of a small Cypris has been recorded in which the spermatozoa are longer than the animal. It is produced in enormous quantities and relatively to other minute cells is extremely tenacious of life. It may retain its vitality in the male organism for a long time after it has become a separate cell, and may exist for lengthy periods in the female organism. ”

“ The Spermatozoan is structurally much different from the ovum, but it also has its nucleus and chromosomes, which carry to the child the transmittable characteristics of the father.
The ovum is usually comparatively large and stationary, and whatever motion is therefore necessary to bring it into contact with the male cell devolves upon the latter, which possesses what is known as a locomotor tail. In addition there are usually many sperms to one ovum, so that the chances are that at least one male cell will reach the egg and effect fertilization, and the beginning of a new life. ”

“ The male nucleus, when it has escaped from the spermatozoid and is observed lying in the cytoplasm at the apex of the egg cell, is of loose, open structure, seeming to have but little kinoplasmic and chromatin matter. The passage to the nucleus is evidently a rapid one, as few stages have been found between the above and the completion of fecundation. ”

“ Returning to the ovum which meets a spermatozoon in the course of its journey down the tube, we find that as soon as a spermatozoon enters an ovum it disappears and is completely absorbed, and, as the ovum in turn is instantly possessed of new powers, through the presence of the male cell, the result of this union is an entirely new cell. ”

“ The Mendelian law of segregation has somewhat lifted the veil in which maturition was wrapped. Every germ cell, being a product of two parent cells, is in one way or other a hybrid. By casting off half of the chromosomes during its maturing, it becomes pure. The gamete is thus always pure and ready for impregnation.
Impregnation.—The process of impregnation is about the same in most many-celled animals. As soon as the head of a single spermatozoön enters the egg-cytoplasm, a new membrane is formed around the ovum which effectually prevents the entrance of any other spermatozoa. ”

“ That of the male is called the spermatozoon (pronounced sper ́ma-to-zoön) . It is sometimes called spermatozoid; the plural is spermatozoa. It is exceedingly small, the smallest of any cell in the body, and has the power to move from place to place. These cells are produced in enormous numbers, and so far as they have been observed under the microscope they differ considerably in power of movement and in perfection of development. Considering their small size, they must make a [163] very long journey to find the ovum; and if they were only few in number, they would rarely succeed ”

“ Spermatozoa originate in the testis as cells, which are filled with granules. After a time, each granule acquires a long appendage, and then the cell has become converted into a bundle of small zoösperms. Development still continues, until finally the thin pellicle on the outside of the bundle is ruptured, thus liberating the young spermatozoa, which speedily complete their full development. The spermatozoön is pure protoplasm, which is the basis of all life, and its power of spontaneous motion is due to this fact. ”