The large fontanelle is made up of four bones and four angles; the small, of three bones and three angles, and are usually easy to differentiate. Furthermore, the difference between these fontanelles is of great importance in labor, since by it the observer is enabled to determine the position of the head. In America, the shape of the head is that of an ovoid with the long diameter anteroposterior (Dolico-cephalic). Thus it happens that when the head is completely flexed, the smallest diameters are presented for delivery. The important diameters of the head, with their measurements and names, are as follows: Nape of neck to center of bregma, 9.5 cm.—Suboccipito-bregmatic diameter. Occipital protuberance to root of nose, 11.25 cm.—Occipito-frontal diameter. Between the eminences of parietal bones, 9.25 cm.—Biparietal diameter. Between anterior ends of coronal sutures, 8 cm.—Bitemporal diameter. The smallest circumference is that of the suboccipito-bregmatic plane, which comes into relation with the brim of the pelvis when the flexion of the head is complete. It measures 27.5 centimeters. The fœtus grows at a definite rate throughout gestation and so regularly that the increase is rarely simulated by any other condition. To find the probable length of the fœtus at any given time, square the month of the pregnancy (up to five) and the result is the fœtal length in centimeters. After the fifth month, multiply the number of the month by five. Thus: 7th month ×5=35 cm., the approximate length of the fœtus at the lunar month.—(Hasse’s rule.) =The Mature Fœtus.=—Although subject to considerable variation, the fœtus at term will weigh about seven and one-fourth pounds, and measure 50 cm. in length. The weight is far more uncertain than the length, and therefore not so reliable as a sign of maturity. To obtain an estimate of the weight of the child at any given month of the pregnancy, the number of lunar months minus 2, is squared and divided by 2, and the result is the average weight of the child at that time in hundreds of grams. Thus: 8th month −2=6. 6×6=36. 36÷2=18, or in hundreds of grams, 1800, the weight of the child.—(Tuttle’s rule.) Differences between the mature and immature fœtus: _Mature_ _Immature_ 1. Skin smooth, plump, pink 1. Skin lax, wrinkled, dull red in covered with vernix caseosa. color; little vernix caseosa. 2. Generous amount of subcutaneous 2. Subcutaneous fat scanty. fat. 3. Hair abundant and from 1 to 2 3. Hair on scalp short. inches long. 4. Lanugo mostly absent. 4. Lanugo present all over body. 5. Nails project from finger tips. 5. Short nails on fingers and toes. 6. Skull bones in contact except 6. Skull sutures open. at fontanelles. 7. Length 50 cm. born. 7. Moves and cries feebly when