If the testes are removed from the normal position and grafted to an abnormal one (or if the testes of another male are trans planted immediately after castration), the organs exert their usual influence on the secondary sexual characters and accessory sexual glands, although their normal nerve connections have been severed. Since, then, the influence of transplanted organs cannot be through intermediation of the nervous system it would seem that it must operate through chemical substances passed into the blood and so into the general circulation. Thus, in experiments upon fowls the testes have been removed and broken up into pieces, which have attached themselves to different parts of the alimentary canal or the wall of the body cavity, and the birds have developed into typical cocks with comb, wattle, etc., male voice, and sexual and combative instincts. Furthermore, the experiments of Steinach and others have shown that the grafting of testes into females whose ovaries had been removed may cause the development of secondary male characters and bring about a partial or complete reversal of sex. There is some evidence that the grafting of additional testicular tissue into the aged may bring about a general rejuvenation, and that if a similar operation is done upon an immature or young animal it may promote an in crease in growth and hastening of maturity (Steinach, Voronoff). Ovariotomy leads to the suppression of the distinctly female characters. If done before puberty the uterus and mammary glands do not develop and the general bodily form tends towards a neutral condition not dissimilar to that of the castrated male. If performed after puberty ovariotomy is followed by cessation of the oestrous or menstrual cycles and the uterus undergoes atrophy in much the same way as occurs normally at the meno pause (climacteric) or time of permanent cessation of reproduc tive activity (in women at from 45 to 5o). If, however, the ovaries (or one of them) instead of being removed are grafted to an abnormal position such as the ventral wall of the body cavity or into a kidney the oestrous cycle is continued and the uterus remains normal. Since the ovary in such a position is without its normal nerve supply it is presumed that its influence on the organism is due to internal secretions passed into the cir culation. The corpus luteum is also believed to be an organ of internal secretion serving the special function of secreting into the blood substances essential for maintaining the raised nutrition of the uterus during pregnancy and for the development of the mammary glands, for if this structure be removed surgically pregnancy cannot continue, the uterus lapses, and the mammary glands fail to develop. The corpus luteum also plays some part in controlling the short or dioestrous cycle, for so long as it is present in its integrity heat cannot occur, but if it is extirpated some days before a new oestrous period is normally due, the period occurs shortly after the operation of removal. Thus Ham mond, by squeezing out the corpus luteum of a cow, has induced oestrus after nine days instead of the usual 19 to 21.
Fertility and Sterility.—The number of spermatozoa dis charged in a normal ejaculation of human semen is estimated at 226,000,00o, whereas the number of ova ejected at one ovulation is usually only one. For mammals generally the number of ova discharged is on an average only slightly more than the average number of young in a litter. It follows that the female rather than the male controls the size of the litter. (On the other hand, Sanders has shown for horses that a reduction in the number of spermatozoa may reduce the chances of the ovum being ferti lized.) The sheep discharges one or two (sometimes three and rarely more) ova at oestrus, but by "flushing" the ewes, that is supplying them with extra or stimulating food before and during breeding, the number may be increased and consequently a higher proportion of lambs obtained. This is an example of the influence of favourable nutrition upon fertility. Too high feeding (resulting in adiposity), however, promotes atrophy of the ova in the ovary and so is conducive to sterility. There is evidence also that steril ity may be due to the absence of certain essential accessory food substances or vitamins (q.v.) and that such a vitamin is present in green food (Evans). Degeneration of ova in the ovary may therefore be due to faulty nutrition of various kinds, but some degeneration is normal.
Sterility may result from coition at an inappropriate time, that is, at too long an interval before or after ovulation, for Hammond has shown in the rabbit that the ova are not capable of being fertilized for longer than four hours after their release from the ovary, and that the spermatozoa in the female passages do not retain their power of fertilization for more than two days. (In the male passage, where they are quiescent, they retain this power for 3o days.) It is probable that in many other mammals the duration of viability of the spermatozoa and ova is not widely different. In animals such as the mare, which has a prolonged oestrus (a week or more), sterile unions may well be due to this cause (Hammond).
Artificial insemination is sometimes successfully resorted to in cases where sterility has been due to an abnormal constriction of the os uteri or to the presence of an acid secretion in the vagina. The practice is to inject the semen directly into the os uteri, thus avoiding the constriction or escaping the action of the abnormal secretion. Walton has found that the semen of rabbits may be kept in a fertile condition in tubes outside the body at a medium temperature for more than five days and that after a journey by post from Cambridge to Edinburgh the spermatozoa con tained therein could still successfully fertilize ova with normal pregnancy as a result.
Fertility, like other characters, is capable of being transmitted from one generation to another. Thus rams which were twin lambs may hand on the tendency to produce twins to the next generation of ewes, and by breeding from rams which were twins the fertility of a flock may be increased.