=By the end of the second month, or eighth week=, the head end of the embryo has greatly increased in size and is about as large as the rest of the body. Bone centres appear in the rudimentary clavicles; the kidneys and supra-renal bodies are formed; the limbs are more developed, webbed hands and feet are formed, the external genitalia are apparent but the sex is not distinguishable. The amnion is distended with fluid, but it is not yet in contact with the chorion; the chorionic villi have become more luxuriant on that part of the chorion resting on the decidua basalis, the future site of the placenta. The approximate weight of the embryo is 4 grams and its length 25 millimetres or an inch. =By the end of the third month, or twelfth week=, centres of ossification have appeared in most of the bones, the fingers and toes are separated and bear nails in the form of fine membranes; the umbilical cord has definite form, has increased in length and begun to twist. The neck is longer, teeth are forming and the eyes have lids. The amnion and chorion are now in contact, and the villi have disappeared excepting at one point where a small, but complete placenta has developed. The embryo is about 9 centimetres long and weighs about 30 grams. [Illustration: FIG. 26.—Diagram showing appearance of fetus at different stages in its development. ] =By the end of the fourth month, or sixteenth week=, all parts show growth and development; lanugo appears over the body; the sex organs are clearly distinguishable and there is tarry fæcal matter, called _meconium_, in the intestines. The placenta is larger, the cord longer, more spiral and also thicker because of the Whartonian jelly which is beginning to form. The fetus is about 15 centimetres long and weighs about 120 grams. =By the end of the fifth month, or twentieth week=, the fetus has both grown and developed markedly. It is now covered with skin on which are occasional patches of _vernix caseosa_, a greasy, cheesy substance consisting largely of a secretion of the sebaceous glands. There is some fat beneath the skin but the face looks old and wrinkled. Hair has appeared upon the head and the eyelids are opening. It is usually during the fifth month that the expectant mother first feels the fetal movements which are commonly referred to as “quickening.” The body is about 25 centimetres long and weighs about 280 grams.