sinuses are provided with unstriped muscular tissue, which can close the inlets from the arteries, and thus shut out the blood. The villi of the foetal placenta, dipping into these uterine sinuses, are covered with a single layer of thin, scaly cells, so that the foetal blood is only separated from the maternal by the walls of

Antero-posterior section through a gravid uterus and ovum of five weeks (semi-diagrammatic). (Allen Thomson.)

a. Anterior wall of uterus. p. Posterior wall of uterus. m. Muscle substance. g. Glandular layer. ss. Decidua serotina. r. Decidua reflexa. v. Decidura vera. ch. Chorion. uu. Uterine cavity. c. Cavity of the cervix.

the capillaries and these thin cells, and thus the interchange of nutrient materials and gas readily go on between them; it is very similar to the conditions of the lung alveoli, where the blood is separated from the air with which it interchanges gases by the cells of the capillary wall and of the lung alveolus.

Though the capillaries of the foetus are in such close relation to the blood of the mother, it must be distinctly understood that there is no direct communication between the vessels of the foetus and those of the mother, and therefore it is not possible to inject the vessels of the mother through those of the fœtus, or vice versa.

The nutrient materials from the maternal blood together with oxygen diffuse through the walls of the foetal capillaries, the effete matter, on the other hand, passing from the capillaries to the blood in the veins which surrounds and bathes these vessels. The placenta increases with the growth of the foetus till shortly before birth, when it is said to undergo a certain amount of degeneration. It is cast out of the uterus after the expulsion of the fœtus with the membranes attached to it. It is, however, only the superficial layer of the maternal placenta (which is intimately connected with the fœtal placenta) that is cast off, the deeper layer remaining in the uterus, and undergoing various changes during the reduction of this organ to its normal size.

After ligature of the umbilical cord, the intimate relationships of the maternal and foetal circulation cease, and it is thought that this causes the inlets of the uterine sinuses to contract, so that when the placenta separates from the uterine walls, the arterioles leading to the sinuses are contracted and possibly occluded with clots. The uterine blood current is thus prevented from escaping into the uterine cavity after parturition, and causing profuse hemorrhage.

The uses of the placenta may be briefly summed up as—

(1) Alimentary, as it supplies the place of the organs of digestion by supplying the foetal blood with nutritive material.

(2) Respiratory, as it performs the function of the lungs, the foetal blood receiving oxygen from the oxyhemoglobin of the mother, to which it gives up its CO,.

(3) Excretory, as it does duty for the kidneys, removing the urea, etc., from the foetal blood.

CHAPTER XXXVIII.

DEVELOPMENT OF THE SPECIAL SYSTEMS.

DEVELOPMENT OF THE VERTEBRAL AXIS.

The earliest evidence of the differentiation of the blastoderm consists in the appearance of the primitive streak which forms the

These are elevations of the epiblast, beneath which the mesoblast is thickened. They arise in front, where they are joined immediately behind the head fold, while posteriorly they diverge, and passing on either side of the primitive groove, gradually

become lost.

Between the two folds is a furrow lined by epiblast, which is called the medullary groove.

The medullary folds growing upward, turn in toward one another, and eventually coalesce at their line of meeting, convert

Transverse section of the embryo of a chick at the end of the first day. (Kölliker.) sp. Mesoblast. dd. Hypoblast. m. Medullary plate. h. Epiblast. P. Medullary groove. Rf. Medullary fold. ch. Chorda dorsalis. uwp. Protovertebral plate. wwh. Division of mesoblast.

ing the medullary groove into a channel-the medullary canal; this union of the folds takes place from before backward.

The medullary canal thus formed lies in the axis of the embryo on the uncleft mesoblast; it is covered in superficially by

(Kölliker.)

rw. Medullary fold. rf. Medullary groove. h. Epiblast. ao. Aorta. dd. Hypoblast. p. Pleuroperitoneal cavity. sp. External plate of mesoblast dividing. uwp. Protovertebral plate.

several layers of epiblastic cells, which also line its walls. The canal is the earliest representative of the nervous centres, and eventually becomes the brain and spinal cord. The front part of the canal, when completely closed in, becomes dilated into a

bulb, thus forming the earliest indication of the brain. The hind part of the medullary groove remains unclosed considerably later than the fore part. It gradually becomes converted into a canal at the tail end, and as it extends backward obliterates the primitive streak and groove, which are lost, and take no permanent part in the formation of the embryo.

Beneath the medullary canal the cells of the mesoblast are altered to form a rod-shaped cellular body, which following the line of the canal lies in the axis of the embryo; this is the chorda dorsalis or notochord.

Supporting the medullary canal on either side of the chorda dorsalis are masses of mesoblast, somewhat quadrangular in section, which are termed the vertebral plates; continuous with

Transverse section through the embryo of a chick on the second day, where the medullary canal is closed. (Kölliker.)

mr. Medullary canal. h. Epiblast. uwh. Cavity of protovertebra utu. ung. Wolffian duct. mp. Mesoblast dividing. hpl. Somatopleure. df. Splanchnopleure. sp. Pleuroperitoneal cavity. dd. Hypoblast. ch. Notochord.

these externally are other thinner masses of mesoblast called the lateral plates.

The lateral plates become divided into an upper part or somatopleure, which is in close relationship to the epiblast, and a lower part, the splanchnopleure, which is next to the hypoblast; the space between these being the pleuroperitoneal cavity. The vertebral plates become separated from the lateral plates by a longitudinal partition, so that on either side of the neural canal is a mass of undivided mesoblast extending laterally toward the divided mesoblast.

In each vertebral plate there appear transverse vertical interruptions at definite intervals, which split the plate up into a