Farm Animals - Facts Concerning Reproduction
( Originally Published 1912 )
The formation of the new animal begins with the union of the material from the male parent with a contribution from the female. Under normal conditions this union takes place within the body of the female shortly after copulation. A general knowledge of the location and construction of the female organs is necessary to a useful understanding of the conditions and processes that have to do with the origin of new individuals. The vulva is the external opening of the female reproductive organs. The vagina is the passage lying immediately inside the vulva and in an ordinary mare isfrom 8 to 12 inches in length. Three or 4 inches from the exterior opening of the vulva is the opening from the bladder. The os (os uteri) or neck of the womb projects into the forward end of the vagina. Its length is about 2 1/2 to 3 inches and owing to the nature of its walls is ordinarily practically closed except at times of breeding or parturition. The womb or uterus is the part that contains the developing embryo. Its rear end opens into the vagina and its forward part is below and to the rear of the kidneys. In unbred mares the main body of the uterus has a length of from 5 to 8 inches.
The ovaries produce the eggs or the female reproductive bodies, and are two in number, one being situated on the right side and one on the left. The ovary of a young mare is reniform in shape, having its greatest dimension of 3% to 4 inches and weighing about 4 ounces. The ovary of the cow is much smaller. It is the function of the ovaries to produce the eggs or ova (singular, ovum) from which the new animal develops after a union with another cell from the male parent. The time at which the ovum is ready to meet this body from the male is marked by evidence of being "in heat." The Fallopian tubes connect the ovaries with the womb and through them the ova are conveyed to the latter.
In the male the testicles are analagous to the ovaries of the female. The structure of the parts provided for the introduction of the product of the testicles into the passage of the female is of slight importance in studying heredity and these organs seldom require attention. The testicles produce very large numbers of bodies, the spermatozoa (spermatozoon, singular). These when discharged from the body during the act of service are contained in a white fluid, alkaline in character, the whole constituting the semen or seminal fluid.
After the act of service a considerable amount of the seminal fluid can usually be found upon the floor of the vagina, though the testimony of some of those who have successfully practiced artificial impregnation of mares is that at least a part of the fluid is present in the uterus soon after copulation. The spermatozoa, which are capable of some motion, work forward through the uterus into the Fallopian tubes. Here they surround the ovum to the interior of which a single spermatozoon penetrates. This union of the male and female reproductive bodies constitutes fertilization. The united ovum and spermatozoon gravitate to the womb, where, if conditions are favorable, growth and development ensue and conception has been accomplished. It is positively known that a new animal is the result of the union of one ovum and one spermatozoon. The reason for the preparation of such a great number of the male bodies, of which only one is essential to reproduction, lies partly in the likelihood of a large proportion of them failing to reach the ovum, or though arrived there having lost their vitality. Any condition that prevents the union of a healthy ovum with a healthy spermatozoon under normal conditions, renders impossible the production of a new animal. Any such obstruction present in the female is known as barrenness. Inability on the part of the male to supply healthy spermatozoa is spoken of as sterility.
Barrenness may result from a diseased condition of the ovaries. Mares and cows that are continuously in heat and fail to conceive are commonly so affected. In such cases no normal ova are produced and treatment is usually unsatisfactory. Excessive fattening during the growing period may derange the ovaries, especially if the elements that support growth are scantily furnished or if exercise and outdoor life are restricted. The os may be so tightly closed as to prevent the entrance of the spermatozoa. This is common in mares that are quite old when first bred and in heifers kept in very high condition. In artificial impregnation some of the seminal fluid is taken from the floor of the vagina and placed within the womb of the same or an-other female, thus overcoming any trouble arising from the condition of the os. In difficult parturitions the os is sometimes lacerated and heals with an enlargement that closes the passage. Acidity of the secretions of the womb also causes barrenness. Reproductive cells, both male and female, require an alkaline medium. If through any diseased condition the fluids of the womb become acid, the spermatozoa perish before reaching the ovum or else the fertilized ovum is destroyed. It is for the remedy of such conditions that the yeast treatment is so commonly recommended for uncertain breeders, but it is not uniformly successful.
Absence of procreative power in the male must be due either to failure to produce normal spermatozoa or failure to convey them to the organs of the female. The first named is the most common cause of sterility. The preparation of reproductive bodies is a deep-seated process and draws heavily on the vitality of the animal. It is essential that a breeding animal be maintained in the best of physical condition by judicious and liberal feeding, reasonable exercise and intelligent management. A few causes underlie nearly all manifestations of sterility. In stallions and in aged bulls temporary sterility sometimes follows a slight and often a radical change of location. It is more often caused by, an excessive proportion of feeds of a fattening character and by a minimum of work or exercise. Excessive service may so decrease the number or vitality of the sperma- tozoa as to produce sterility. In no event can a second service overcome obstacles to conception in either parent. One satisfactory service furnishes a superabundance of spermatozoa; other services can only exhaust the vitality of the male. If wrong conditions are indicated or suspected a remedy should be used before mating is allowed.
It is thought that the breeding of sows late in the period of heat renders more certain the presence of active spermatozoa at the time the last ova leave the ovaries, thus ensuring fertilization of all ova produced. In as much as the number of spermatozoa is so great it is evident that the number of young to a litter must be controlled by the female unless the male be so seriously overtaxed as to lower the number or vitality of spermatozoa, or unless mating occurs at a time too far removed from the time of production of the ova, so that either or both perish before fertilization is accomplished. The number of rudimentary eggs or ova present in the ovary is much greater than the number that can possibly be discharged in a lifetime. Any condition which would augment the production of ova in sheep or swine would of course add to the number of young produced at a birth. A normal, well nourished dam might be ex-expected to mature more ova at one time, but no direct influence can be brought to bear upon this function. The egg cell or ovum is expelled from the follicle of the ovary in which it was prepared at the time of the evidence of "heat" and ordinarily before copulation occurs. It is not known just how long an ovum retains its life after being discharged, but it is probably a considerable time. With the female organs in an entirely normal state it is believed that the spermatozoa may remain active for several days after their introduction. In one case, with a rabbit, spermatozoa are known to have functioned ten days after copulation.
The statement was made in the preceding chapter that the new animal develops from two single germ cells, an egg or ovum from the dam and a sperm cell or spermatozoon from the sire. It is natural to entertain the thought that the dam, during the months of gestation in which she carries and nourishes the developing offspring, has opportunity to give to it a stronger impress than was received from the sire. Undoubtedly much depends upon the nourishment afforded the foetus by the dam and this important feature is re-served for later discussion. The actual connection between the membranes enveloping the foetus and the inner surface of the womb allow absorption from the maternal into the foetal circulation of material for the support of growth, but there can be no blood current from one to another. Furthermore, we know that while blood carries building material to the various parts of the body the ability to shape the material rests, not in the blood, but in the contents of the cells that make up the part.
Observation clearly corroborates the idea that the dam has no opportunity to dominate the make-up of the off-spring more than is enjoyed by the sire; indeed the claim seems well founded at times that the sire's influence exceeds that of the dam. It is clear that aside from feeding the embryo animal the entire determination of what that embryo is to become resides within the two cells with the union of which the new life was inaugurated, and the one from the sire contains active material equal in amount and determining power to that in the dam's contribution to the embryo offspring. Any particular points or conformation or fitness for special agricultural requirements that are to characterize any descendant of a long line of carefully selected ancestry must have its representation in one of these two sexual cells. Clearly then, it is of first importance that we fully understand the nature and behavior of these germ cells and their relation to the parent body.