already larger than the umbilical vesicle, and resembles the embryos of other animals. It is much curved, has still the branchial arches, a distinct tail, and only a trace of extremities. The umbilical cord is quite short, and the opening in the abdominal walls not very large. The accumulating liquor amnii commences to separate the body of the embryo from the amnion.

Second month.-In this period the ovum undergoes very great changes. It acquires the size of a hen's egg; the embryo is from 2 to 3 cm., or 98-1.2 inches long, and weighs about 4 gram., or 62 grains. The embryo has already its permanent shape, the extremities have distinctly separated into their three portions, the umbilical cord is longer, and the intestine still extends into it. In the clavicle and inferior maxillary bone the first centres of ossification appear.

Third month. The ovum is of the size of a goose's egg; the embryo is from 7 to 9 cm., or 2·7 to 3.5 inches, long, and weighs from 5 to 20 gram., or 77.5 to 310 grains. The intestines are retracted from the umbilical opening, and centres of ossification are found in most of the bones; the fingers and toes with their nails are distinctly discernible, and the external genitals begin to assume the sexual distinction.

Fourth month. The fœtus is from 10 to 17 cm., or 6.6 inches, long, and weighs as much as 120 gram., or 1860 grains. The sex can now be distinctly recognised.

Fifth month.-The fœtus is from 18 to 27 cm., or 7 to 10.5 inches, long, and weighs on an average 284 gram., or 4402 grains. The skin is less transparent. Hair is growing on the head, and lanugo over the whole body.

Sixth month.-With a length of from 28 to 34 cm., 11 to 13 inches, the fœtus has a medium weight of 634 gram., or 9827 grains. Fat begins to be deposited in the subcutaneous connective tissue, but it is still small in amount so that the skin is as yet quite wrinkled. The head is disproportionately large; the fontanelles and sutures are widely opened. A fœtus born at this time makes inspiratory movements, moves its limbs, but dies in a very short time.

Seventh month.-The foetus is from 35 to 38 cm., or 137 to 15 inches, long, and weighs 1218 gram., or 42 oz. The eyelids are separated, the whole body is on account of the scanty development of fat, thin, the skin red and covered with vernix caseosa. A fœtus born between the twenty-fourth and the twenty-eighth week sometimes moves the limbs with some strength, but cries with a feeble voice, and even though very careful attention is paid to it, almost always dies in the first few hours, or at the most days, after birth.

Eighth month. The foetus is from 39 to 41 cm., or 15 to 17 inches, long, and weighs on an average 1567 gram., or 49 oz. The pupillary membrane is disappearing, and the external skin maintains its red colour. The foetus is still thin, and has a senile aspect. However, children born at this time (twenty-eighth to thirty-second week) may under favorable circumstances be kept alive, but they very frequently die, and from slight causes.

Ninth month. The length of the foetus is 42 to 44 cm., or 16 to 17 inches, its medium weight 1971 gram., or 62 oz. The great

development of fat contributes to give the body a rounded shape, and the face loses its wrinkled appearance. Children born in the thirty-second and thirty-sixth week show a far greater mortality than those born at term, but under favorable circumstances they are, as a rule, kept alive.

Tenth month. In the first weeks of the tenth month the fœtus is from 45 to 47 cm., or 17.5 to 18.5 inches, long, and weighs 2334 gram., or 82 ozs. The lanugo gradually disappears, but is still almost everywhere distinctly visible, and most so on the shoulders. The nails have not yet reached the tips of the fingers, the cartilages of the ears and nose are soft, the skin is still red, but smooth and full. Towards the end of the tenth month the fœtus acquires all the characteristics of the mature child and can no longer be distinguished from it.

2. The Foetus at Term

The characteristic marks of a mature foetus are the following: It is, on an average, 51.2 cm., or 20 inches, long, and weighs 3275 gram., or 63 lbs. The skin is white; the fine lanugo is visible only on the shoulders, and has disappeared from every other part; the child is more or less covered by vernix caseosa (a whitish grease, formed of thrown-off epithelium, fine lanugo, and the secretion of the sebaceous glands). The hair of the head is usually dark, from one to one and a half inches long; the cartilages of the ear and nose feel hard; the nails also are usually hard, horny, and project at least in the upper extremities over the tips of the fingers. The umbilical cord is inserted somewhat below the middle of the body. In males the testicles can be felt in the thick wrinkled scrotum; in females, the labia majora are closely approximated; often, however, the labia minora are visible. The cranial bones are hard, they lie close to cach other (the sutures are narrow), and the ossific centre of the lower epiphysis of the femur is about cm., or inch, in its greatest diameter. The child cries immediately after birth with a loud and powerful voice, and actively moves the extremities. It also passes urine and meconium. The latter is of a blackish or brownishgreen colour, and consists of mucus, intestinal epithelium, bile, epidermic scales, and lanugo (Förster, 'Wien. Med. W.,' 1858, No. 32). The head of the child, being its most voluminous and most unyielding part, is of special importance in the mechanism of parturition, and therefore requires a more detailed consideration.

The face appears to be very small in proportion to the cranium. This latter is formed by the two frontal bones, the two parietal bones, the squamous portion of the occipital bone, and at the sides. by the temporal bones and the large wings of the sphenoid. All these bones have not grown to one another, but are separated from each other by spaces, the so-called sutures. The following sutures are distinguished:

1. The frontal suture, between the two frontal bones.

2. The sagittal suture, between the two parietal bones.

3. The coronal sutures, one on each side between a parietal and a frontal bone.

4. The lambdoidal sutures, one on each side between an occipital

and a parietal bone. The serrated temporal sutures, by which the squamous portion of the temporal bone is connected with the parietal bone on each side, cannot be felt on heads covered by the soft parts, the temporal muscle especially covering them.

At the place where the two coronal, the frontal, and the sagittal sutures meet, the bones present greatly rounded-off angles, and a large space is formed in the bony walls of the head, which is called the large fontanelle. It has the shape of a trapezium; its angle, formed by the meeting of the two frontal bones, is much more acute than that between the two parietal. The small fontanelle does not consist of a true membranous gap, but its place is marked only by the meeting of the sagittal suture with the lambdoidal sutures. At the ends of the lambdoidal sutures, close to the mastoid portions of the temporal bones, there are two distinct intervals in the cranium, the lateral fontanelles-font. Gasserii.

To determine the size of the foetal head, the following measurements are taken :

1. The antero-posterior diameter, between the glabella and the most prominent point of the occiput, 4-62 inches, or 113 cm.

2. The larger transverse diameter; the greatest distance in a transverse direction, measures 3.64 inches, or 91 cm.

3. The smaller transverse diameter, the largest transverse distance between the two coronal sutures, 3.14 inches, or 8 cm.

4. The oblique or large diameter, from the chin to the most distant point of the cranium close to the small fontanelle, measures 5.31 inches, or 131 cm.

5. The vertical diameter, between the vertex and occiput, which in the living child cannot be accurately measured, from 3-74 to 3.93 inches, or 9 to 10 cm. The circumference of the head is 341 cm., or 13.58 inches.

There are, however, considerable individual deviations from the above-given average measurements; generally the heads of male children are a little larger than those of female, and the children of older pluriparæ have greater measurements than those of young primiparæ.

3. The Nutrition and Circulation of the Foetus

The nutrition of the fecundated ovum is at the first carried on by mere osmosis. The nutritive irritation, which the ovum embedded in the uterus exerts, causes an increased supply of nutritive material and the structureless primitive villi of the chorion enlarge, as Kiwish says, "the surface for absorption, and are permeable also for liquids on account of their delicate formation. The activity thus roused immediately converts a part of the absorbed fluid in the ovum into cells, that is, the embryo, as it were, crystallizes out of it."

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As soon as the primitive vascular system is developed the omphalo-enteric vessels absorb the nutritive material contained in the vitelline vesicle and bring it to the embryo.

The most important change, however, takes place when the allantois has brought the foetal vessels to the serous membrane which becomes the exochorion. Their great development at the

placental insertion and the immediate prolongation of the villi of the chorion into the maternal sinuses allow of such an extensive interchange of the gases and liquids of the two bloods that the respiration and nutrition of the foetus become thereby possible.

[It is not exactly known what kind of interchange this is; however, it appears almost certain that the red blood-corpuscles of the fœtus give off carbonic acid and receive oxygen, and that the plasma of the foetal blood so exchanges its constituents with those of the mother's blood that the final products of the tissue metamorphosis of the fœtus enter the blood of the mother, and the highly organized combinations which the maternal blood has derived from the chyle enter the foetal blood. An immediate and direct mixing of the two kinds of blood nowhere occurs; they are separated by the epithelium lining the villi of the chorion. Reitz (Centralblatt f. d. Med. Wissens.,' No. 41, 1868, p. 655) after injecting a gravid rabbit with cinnabar found the colouring particle also in the blood of the fœtus, and especially in the capillaries of the pia mater. According to Cohnheim's observations on the migration of the white bloodcorpuscles from the vessels, such a passage, undoubtedly connected with the white blood-corpuscles, is easily intelligible, and consequently cells of the maternal blood would also enter into the circulation of the fœtus.]

The nutrition of the foetus is almost exclusively carried on by the placenta, and not by the liquor amnii which chiefly contains products of a retrogressive metamorphosis and only very little albumen. As regards the respiration of the foetus, Pflüger correctly draws attention to the less amount of movement (heat and work) performed by the foetus, and the consequently smaller quantity of oxygen required by it then as compared with extrauterine life. The foetus is suspended in a medium the heat of which corresponds to about that of its own blood, does not receive into its intestinal canal nor into its lungs cold substances to be heated therein, nor does it lose heat by radiation or by evaporation, either at the surface of its body or through the lungs. The work of the muscles, too, is proportionately small. Active movements are easily executed in a liquid the specific gravity of which is the same as that of its own body. The respiratory muscles are not in action, and the heart alone is powerfully at work. Nevertheless the fœtus does respire-that is to say, there is a consumption of oxygen; we conclude that this is so from the known fact that an interruption of the foetal circulation, when the placenta is not replaced by the lungs, always causes death, and this occurs at a time when the want of nourishment cannot possibly prove fatal; and again on examining the bodies of those who have died from such a cause there are the unmistakable signs of death from asphyxia.

It has also been proved that the foetus makes respiratory movements when its communication with the placenta is interrupted (Vesal); and, on the contrary, the new-born child ceases to inspire, becomes apneic, when oxygen is supplied to it in another way than by the lungs (Mayow).

From this we may conclude that the foetus inspires as soon as it feels the want of oxygen, and that during intra-uterine life oxygen is supplied by the placenta. Pflüger has also experimentally

shown from the colour of the blood that the foetus consumes oxygen.

The circulation of the blood in the foetus takes place in the following way :-The umbilical arteries, the principal terminations. of the iliac arteries, convey the blood through the umbilical cord into the placenta, and in the chorion villi the inter-changes with the maternal blood occur. The umbilical vein collects the blood suitable for the nutrition and respiration of the foetus from the placenta, and brings it through the umbilical cord to the liver (for the sake of brevity we shall call this blood arterial, and that which has circulated in the foetus venous, although neither of them exactly corresponds to the arterial and venous blood of extra-uterine life). A part of it circulates through the liver, another portion passes through the ductus venosus Arantii directly into the inferior vena cava, and both these mixed enter the right auricle, together with the venous blood returning from the lower half of the body. At an earlier period of fœtal life the inferior vena cava opens opposite the septum of both auricles into the left as well as into the right auricle, but its blood chiefly enters the left on account of the greater development of the Eustachian valve on the right side of the septum. In the second half of fœtal life the Eustachian valve becomes smaller, whilst, through increased development of the valve of the foramen ovale, the blood current of the inferior vena cava is more and more exclusively directed into the right auricle. At that time, therefore, its blood is partly mixed with that of the superior vena cava, so that mixed blood enters the right ventricle as well as the left auricle. The greater part, however, of the venous blood, which returns by the superior vena cava from the upper half of the body, enters the right ventricle and is hence forced into the pulmonary artery. But since a pulmonary circulation (in the true signification of the word) does not yet exist, only about half of the blood from the right ventricle enters the pulmonary artery, whilst the other half is poured through the ductus Botalli into the descending aorta. This latter contains blood from three different sources. The left auricle receives only a relatively small quantity of blood from the yet undeveloped pulmonary veins. It obtains its chief supply through the foramen ovale, through which, as stated above, the current of the inferior vena cava passes; this is chiefly arterial blood, though not quite unmixed. Hence this blood passes through the left ventricle into the first portion of the aorta, so that the carotid and subclavian arteries are supplied chiefly with arterial blood. Beyond the origin of those arteries the more venous blood of the right ventricle, and chiefly derived from the superior vena cava, enters through the ductus Botalli into the descending aorta. Therefore the umbilical vessels and the lower half of the trunk are supplied with more venous blood than arterial, chiefly by the force of the right ventricle.

These conditions, however, are at once altered at the birth of the child. By the first respiratory movements the lungs are expanded and thereby the pulmonary arteries dilated. Hence the blood of the right ventricle is exclusively forced into the pulmonary artery. In consequence of this a far larger quantity of blood, after being