Leeuwenhoek believed them to be the beginnings of future animals--that they are of different sexes, upon which depends the future sex of the foetus. Be this as it may, it appears to be admitted on all hands that the animalculae are present in the semen of the various species of male animals, and that they cannot be detected when either from age or disease the animals are rendered sterile. "Hence," says Bostock, "we can scarcely refuse our assent to the position that these animalculae are in some way or other instrumental to the production of the foetus." The secretion of the semen commences at the age of puberty. Before this period the testicles secrete a viscid, transparent fluid, which has never been analyzed, but which is doubtless essentially different from semen. The revolution which the whole economy undergoes at this period, such as the tone of the voice, and development of hairs, the beard, the increase of the muscles and bones, etc., is intimately connected with the testicles and the secretion of this fluid.* "Eunuchs preserve the same form as in childhood; their voice is effeminate, they have no beard, their disposition is timid; and finally their physical and moral character very nearly resembles that of females. Nevertheless, many of them take delight in venereal intercourse, and give themselves up with ardor to a connection which must always prove unfruitful."** * Nichol's "Human Physiology," pp. 256-257. --Publishers' note ** Magendic's Physiology.--Author's note. The part performed by the female in the reproduction of the species is far more complicated than that performed by the male. It consists, in the first instance, in providing a substance which, in connection with the male secretion, is to constitute the foetus; in furnishing a suitable situation in which the foetus may be developed; in affording due nourishment for its growth; in bringing it forth, and afterward furnishing it with food especially adapted to the digestive organs of the young animal. Some parts of this process are not well understood, and such variety of hypotheses have been proposed to explain them that Drelincourt, who lived in the latter part of the seventeenth century, is said to have collected two hundred and sixty hypotheses of generation. It ought to be known that women have conceived when the semen was merely applied to the parts anterior to the hymen, as the internal surface of the external lips, the nymphae, etc. This is proved by the fact that several cases of pregnancy have occurred when the hymen was entire. The fact need not surprise us, for, agreeable to the theory of absorption, we have to account for it only to suppose that some of the absorbent vessels are situated anterior to the hymen--a supposition by no means unreasonable.