Farm Animals - The Offspring During Gestation
( Originally Published 1912 )
The successive stages of development from a fertilized ovum to a fully formed body occur with striking regularity and uniformity. These changes, however, are of more immediate interest to the embryologist than to the practical student of heredity. But even the latter should not lose sight of the significance of one of the stages which was referred to in Chapter VI, the separation of an amount of the germ plasm or hereditary material to be preserved in the ovaries or testicles as the case may be, for the production of the germ cells of the succeeding generation. This idea renders more easy the appreciation of the importance of good ancestry.
It is known that in most animals some days elapse before the fertilized ovum becomes attached to the uterus to be sustained from the blood circulation of the dam. During the interval the changes that occur are supported by the considerable amount of food material carried by the ovum or egg-cell. It was noticed in Chapter IV that there is a natural tendency to suppose that the very intimate contact existing between the foetus and the dam through such a long period of time affords her extraordinary opportunity to imbue the young with her own qualities to the exclusion of those of the sire. The indisputable facts concerning the physical basis of heredity show clearly that no matter how plausible such an idea may seem it is entirely erroneous. This in no way detracts from the importance of maternal vigor and good care and feeding to render certain that there shall be present in the circulation of the dam all those elements requisite to the maximum growth of the foetus. The attachment of the embryo to the uterus is entirely analogous to the rooting of the germinated seed, after that point everything is conditional upon food supply. The good seed contains unusual possibilities, but is helpless and useless in the absence of the material with which to build to the plan it contains.
Numerous instances have been cited to substantiate the claim that certain conditions may so impress the dam at the time of conception or during pregnancy as to cause representative conditions in the offspring. The writer has known of a Galloway cow that dropped an off-colored calf. The owner with apparent seriousness attributed the occurrence to the fact that before the cow was bred the herdsman had allowed the bull to serve a neighbor's family cow the color of which was conveyed to the calf in question. It is unusual to claim such trans-mission through the sire, but credence is too often given to the possibility of such influence through the dam. It is related that the celebrated Angus breeder, McCombie, attributed some of his success in ridding his herd of the tendency to throw white spots and off-colors to his having painted all his barns and fences in a solid black color so as to impress his breeding animals.
That impressions upon the mind of the pregnant mother are reflected in the offspring is completely impossible of explanation on a physiological basis. It is true we are very unfamiliar with the nervous system, but to suppose that even violent mental impressions could originate a substance that would so derange inheritance as to produce a serious change in one organ or part while others are not affected would be going very far to ex-plain even, undisputed facts. It is also well to bear in mind that the circulation of the dam does not pass directly through the foetus, but that the latter has its own system of circulation which is replenished by filtration through the numerous cotyledons that connect the inner maternal and the outer foetal membranes.
That a mental impression could set up an action that would be conveyed to a specific part of the foetus is unthinkable. There doubtless have been cases where animals were born with some deformity or malformation corresponding to a condition that impressed the dam during pregnancy. But such . cases are so very rare as to compel us to class them as coincidences. For every such case that can be cited, there are thousands of others in which the same or equally likely influences were exerted with no result. If it were true that visual impressions could be conveyed to the offspring, breeding would be chaos. Colts would have the color of the cattle or swine, the calves conceived in summer would be of a green color and those of winter would reflect the varied hues of the surroundings of the yards and the interior of the stables. Speculation and discussion in regard to these very rare coincidences diverts attention from the tangible basis of heredity which alone yields suggestions that can safely be carried into practice.
The embryo must not, however, be regarded as unsusceptible to the effects of the mother's condition. Although maternal impressions can not be directly conveyed to a specific part or organ of the foetus, regard must be had for the fact that severe nervous disturbance occasioned by fright or anger may interfere with nutrition. Authenticated cases give best of grounds for believing that anger may so derange the nerves and the organs they control as to cause an abnormal and injurious condition of the milk. Since milk is a blood product it is reasonable to suppose that the same malnutrition may also extend to a foetus in the uterus and cause a partial or complete interruption of nutrition of the foetus and death or expulsion or both. Such possibilities suggest the general precautions against allowing infoal mares to be in sight of blood, and against the feeding of damaged feed to any stock carrying young. Strange and mysterious marks and conditions may also be the result of the displacement of the foetus and pressure or entanglement of parts in the cords in such a way as to cut off the circulation to the part. Such conditions suggest the protection of the dams from undue exertion and rough treatment which may also cause abortion or the death of the young.
While it is true that no regard need be had for direct influence of maternal impressions and that reasonable treatment will preclude accidental abortion, still the time of gestation affords important op portunities to second the efforts put forth in selection. Reproduction is a normal function, and only normal treatment of the parents is necessary to insure its successful accomplishment. While as with plants improper or insufficient nutrition may not produce specific effects, at the same time the foetus can complete no development for which the material to build with is not forthcoming from the dam's circulation. The hereditary material represents the ability and powers of architects, but the most expert architect is just as helpless without necessary materials as is the man surrounded by material but without masons or carpenters.
The securing of maximum development before birth has a very important relation to the outcome of any mating. The necessity of liberal feeding of the mother to insure a plentiful supply of milk is easily recognized, but the beginning of suckling, while it is a vital transition to the offspring, for the dam marks only a changed method of nursing. The nourishment of the offspring prior to birth may have just as strong an influence upon its final development as that furnished after it enters upon a separate existence. Any meagerness of the feeding during pre-natal days impairs and restricts the development of all the organs. Under 'favorable circumstances such under-development may be overcome by careful feeding after birth, but such procedure consumes time that might have been utilized in making progress toward maturity and never can fully compensate for curtailment at the more opportune time. Though prematurely born animals that are exceptionally well tended sometimes mature well, cases are not infrequently met with in which lack of vitality and many serious forms of weakness are traceable to under development at birth.
The support of the growth of the foetus through the feeding of the dam must be considered as being accomplished only after the demands for her own sustenance and whatever may be exacted in the way of milk or labor have been satisfied. Nor is a fat condition of a pregnant female evidence that the feeding is judicious. The fat producing feeds are not what is chiefly required by the growing young. The in-crease consists mainly of bone, muscle and body tissue, and must be furnished in the dam's ration. Liberal supplies of unwisely selected feeds are not of themselves a guarantee of the most desirable results. Unhealthy stabling, poor ventilation or restricted exercise may preclude the most healthy and efficient condition of the dam and thus hinder the young from accomplishing what was made possible in planning its inheritance.
The bovine embryo at the end of the third month has a length of about 5 1/2 inches. At the end of the fourth month the length is in the vicinity of 10 inches and the weight 4 1/2 pounds. Up to this point the tax upon the dam has not been severe and might be supported while she devoted consider-able of the food products to labor, milk and growth. During the fifth month length and weight both increase by about 50 per cent. It can readily be seen, then, that in the case of a calf weighing 80 pounds at birth there is an increase in weight of over 70 pounds during the last four months. This weight consists nearly altogether of bone, muscle, and other tissue. It represents a gain through growth of over half a pound daily, that must be supported from the dam's feed. If her ration is lacking in the required elements the young must most certainly suffer. If she is so fed and handled as to continue a heavy milk flow that demand is not unlikely to be sup-plied at the expense of the foetus. If her growth is in-complete, and the ration not liberal enough to meet the needs of two individuals, one and probably both will suffer. One very successful breeder and exhibitor with whom the writer was acquainted stated that his best calves in a surprisingly large proportion of cases were from cows that had missed breeding the previous year. This breeder considered that the rest permitted the cow to. be in the best possible condition to nourish her young both before and after its birth. Reasonable exercise at liberty or at work may promote the growth of a foal through the general health of the mare, but severe labor or even modest labor, when only fat producing foods are fed, can be exacted only by sacrificing in some measure the natural vigor needed by a well bred animal.
Before proceeding to a consideration of the interests of the off spring one other topic may be treated. Along with wonderful tales of the effects of maternal impressions there are recorded in-stances which are taken to show the residual influence of a sire upon other later offspring produced by the female to the service of a different male. This is designated as telegony, or the influence of a previous impregnation. Belief in such a supposed phenomenon is illustrated in the idea that a mare raising a mule colt, though mated with a male of her own kind the following year, will produce a foal exhibiting characteristics of the ass. The majority of breeders have no regard whatever for any possible influence of earlier sires because their experience and observations do not so suggest. It used to be claimed that telegony was operative in dog breeding, but the number of dog breeders who believe in it is rapidly decreasing. Supposed occurrences in this class of stock can be explained on other grounds. There is no satisfactory physiological explanation of telegony. The facts do not suggest that such a thing exists ; it merits no practical or speculative consideration by breeders.