SEX IN THE EMBRYO AND FETUS. It is impossible to determine the sex of the embryo during its early stages. During the fourth week the first traces of the sexual glands appear, but not until the fifth week can the sex be determined even by the microscope. If the embryo is to become a male, certain ducts are transformed into convoluted tubules, and each is attached to the testes which have been formed from the genital nucleus. If the embryo is to become a female, the ducts join to form the uterus and vagina, other portions being transformed into the fallopian tubes and connecting with the ovaries which have been formed otherwise. The outer genitals appear in the early stages of the embryo, but there is no apparent distinction between the sexes, the external organs being the same in all cases, and consisting of a small tubular organ with a small lateral fold of skin on either side. Later, in the male, a groove appears on the under side of this primitive organ, thus forming the urethra, the scrotum being formed from the folded skin at the side. In the female, the primitive organ ceases to develop as in the male, and thus becomes proportionately smaller, and evolves into the clitoris of the female; the two lateral folds, on each side, being transformed into the labia majora, or "outer lips" of the female external genitals. POSITION OF THE FETUS. During the period of gestation the fetus lies "curled up" in the bag of the amnion. The head is usually relaxed and inclined forward, the chin resting on the breast; the feet are bent up in front of the legs, the legs bent up on the thighs, the knees separated from each other, but the heels almost touching on the back of the thighs; the arms bent forward and the hands placed between them as though to receive the chin between them. The folded-up fetus forms an oval, the longest diameter of which is about eleven inches at its greatest stage of growth. Nature here shows a wonderful ability to pack the fetus into as little space as possible, and in such a position as to protect it from injury, and to discommode the mother as little as possible. The following interesting statement made by Helen Idleson, M. D., in a European medical journal several years ago, gives a very clear idea, expressed in popular terms, of the appearance and characteristics of the embryo or fetus in the various stages of its development: