The ovarian changes (at any rate in the bitch) are also similar in both pregnancy and pseudo-pregnancy. The Graafian follicle, after parting with its ovum, becomes converted into the corpus luteum or "yellow body," so called on account of a pigmented fat (lutein) formed inside it. The yellow body is formed by the rapid hypertrophy of the cells surrounding the wall of the f ollicle; this is so great that the individual cells increase in size 16 or 20 times. This structure, which plays an important part during pregnancy, lasts throughout that period (and correspondingly during pseudo-pregnancy) and then undergoes degeneration.
In polyoestrous animals there is a succession of oestrous periods within a single breeding season, that is to say, that if coition does not take place at the first oestrus, or if for some other reason the ova discharged at ovulation are not fertilized, the animal, instead of experiencing a prolonged pseudo-pregnancy followed by an anoestrum (as with the bitch), undergoes a short period of apparent quiescence, called by Heape the dioestrum, and then "comes on heat" again. Thus with the sheep, the ewe, if she fails to become pregnant at her first oestrus, "comes back to the ram" (as the shepherds say) after about 15 days, and if she again fails, may experience a third oestrus after another 15 days, and so on for a succession of cycles until the breeding sea son is over or the ewe succeeds in becoming in-lamb. This short (or dioestrous) cycle in the sheep is therefore 15 days. The number of dioestrous cycles which the animal is capable of ex periencing depends partly on the breed and partly on the environ ment, whether favourable or otherwise. Among sheep of all , breeds there is a complete gradation between the monoestrous condition of certain wild varieties and the extreme of polyoestrum exhibited by certain merinos, in which there may be no anoestrum (even in the non-occurrence of pregnancy) but (in the absence of the ram) an unbroken succession of dioestrous cycles which last the whole year. Many wild animals (e.g., rodents) are poly oestrous and the dioestrous cycle may last for only a few days. In polyoestrous animals ovulation typically occurs during oestrus and is followed by the formation of the corpus luteum.
The time of persistence of this structure varies according to whether or not pregnancy occurs. In the absence of pregnancy the corpus luteum persists for the duration of the dioestrum and then begins to degenerate as if to make way for the ripening of a fresh batch of follicles in the ovary and a new oestrous period. If, however, pregnancy takes place the corpus luteum continues in the ovary until parturition as in monoestrous animals. In reality, the dioestrum, instead of being a period of complete rest, is of the nature of a very abbreviated pseudo-pregnancy, and the uterus undergoes some growth changes in association with the presence of the corpus luteum in the ovary.
In man there is typically no anoestrum (except among the Esquimaux in winter) and the menstrual cycles, each lasting about a month, correspond to the dioestrous cycles of the poly oestrous lower mammal The actual menstrual phenomena probably represent the degenerative changes at the end of an abbreviated pseudo-pregnancy (or dioestrum) telescoped into the prooestrum of a new cycle. Ovulation takes place most com
monly about the r 8th day after the beginning of the menstrual flow but it may occur at other times, though rarely, in the week or ten days before the beginning of menstruation.
In some animals (rabbit, ferret) ovulation only takes place after coition. The actual process can be demonstrated in an anaesthetized rabbit whose ovaries have been exposed to view (Hammond). It is probable that in man also ovulation may sometimes require the additional stimulus set up by coition. In most domestic animals (bitch, sow, ewe, cow, mare) ovulation takes place spontaneously at or about the time of oestrus.
The Testis and Ovary as Organs of Internal Secretion.— It has been mentioned that the testis, besides producing the spermatozoa, is also an organ elaborating an internal secretion which is discharged into the blood. A similar statement may be made about the ovary. The evidence for these conclusions falls under three heads, (I) the effects of removing these organs (castration), (2) the effects of transplanting the testis or ovary into animals whose own gonads have previously been removed, and (3) the result of injecting tissue extracts prepared from testes or ovaries.
The general effect of castration in all vertebrate animals is to prevent the development of the secondary characters of sex, that is, of those characters which, while correlated with the sex in question, are not directly concerned with reproduction. This statement applies to ovariotomy or the extirpation of the ovaries in the female as well as to castration in the male. It is essential, however, that this operation should be performed early in life to have its full effect. It not only ensures permanent sterility (whenever it is done) but if performed on the young stops the development of superficial sexual characters as well as the accessory reproductive organs (prostate gland, etc.). Thus, in man, castration prevents the growth of hair on the face and various parts of the body and arrests the enlargement of the larynx and the consequent deepening of the voice normally char acteristic of puberty in the male. In stags castration inhibits the growth of the antlers and in those breeds of sheep which are horned in the male and hornless in the female it prevents develop ment of the horns ; moreover, the horn growth is arrested at any stage of development at which castration is performed. With fowls, castration is followed by an arrest of the development of the erectile structures about the head (comb, wattles, etc.). Cas tration has been practised on the domestic animals from the earliest times, for it improves the quality of the flesh and favours fattening in meat-producing animals and is conducive to a greater tractability in working animals since the disturbing effects of sexual desire no longer occur.