of tessellated cells arises from this layer, viz., the endothelium, forming the surface of all serous membranes. From the hypoblast spring the epithelial lining of the alimentary canal, that of the glands which are diverticula from it, and of the lungs; it also forms the lining membrane of the allantois and yolk sac.

The blastoderm of the hen's ovum, which is comparatively easily studied, consists of a small, clear, central portion, called the area pellucida, from which the body of the chick arises. Surrounding the area pellucida is a much larger zone, which appears less transparent; this, the area opaca, is devoted to the formation of the membranes.

Diagrammatic longitudinal section through the axis of an embryo chick. (Foster and

Balfour.)

N. C. Neural canal. Ch. Notochord. D. Foregut. F. So. Somatopleure. F. Sp. Splanchnopleure. Sp. Splanchnopleure forming the lower wall of the foregut. Ht. Heart. pp. Pleuroperitoneal cavity. Am. Amniotic fold. A. Epiblast. B. Mesoblast. C. Hypo

blast.

The embryo is developed from the rest of the blastoderm in the following manner. At the front of the area pellucida a fold, or dipping in of the blastoderm takes place; this consists of a projecting part above and a groove below, and constitutes the cephalic, or head fold. The upper projecting portion of the fold tends to grow forward, while the groove grows gradually backward. Later on, another fold appears at the posterior part of the area pellucida; this is the tail fold. At the sides of the area pellucida folds appear, which tend to grow downward and inward so as to reach the under surface of the blastoderm and unite with the head and tail folds.

By the approximation of all these folds a canal is formedthe embryonal sac-which is closed above by the main portion of the area pellucida, in front by the head fold, behind by the tail fold, at the sides by the lateral folds, while below it is open to the vitellus. This canal ultimately becomes subdivided into an inner tube, the alimentary tract, and an outer one, which forms the body walls, the final place of union of the folds being marked by the umbilicus. It must be clearly understood that these primary folds which form the embryo include in their layers the epiblast, the whole thickness of the mesoblast, and the hypoblast, whereas the folds giving rise to the membranes do not comprehend all these layers.

FORMATION OF THE MEMBRANES.

(1) The Amnion.-The mesoblast around the embryo becomes thickened, and is split into two distinct layers; this cleavage is at first confined to the neighborhood of the embryo, but gradually spreads over the whole blastoderm.

The upper of these two layers of the blastoderm receives the name of the somatopleure, and is engaged in the formation of the body walls of the embryo and the amnion. The lower one is called the splanchnopleure, and forms the walls of the alimentary canal, the allantois, and the yolk sac. The space intervening between these layers is called the pleuroperitoneal cavity. At a point in front of the cephalic fold, an upward projection of somatopleure takes place, conveying with it the overlying epiblast. Along the sides of the embryo and behind the caudal fold, projections of the somatopleural mesoblast and epiblast also occur. Thus folds are developed consisting of somatopleural mesoblast and of epiblast, which tend to grow upward and meet over the back of the embryo. These are the amniotic folds, and each presents two surfaces, one looking toward the embryo and the other toward the vitelline membrane. As they meet over the back of the embryo the folds become fused, the membranes looking toward the embryo joining to form the amnion proper, while those next the vitelline membrane unite to form the false amnion, which, separating from the amnion proper, retires toward the

vitelline membrane, with which it unites to form the primitive chorion.

The proper amnion then is a sac formed of an outer layer derived from the mesoblast and an inner layer derived from the epiblast. The false amnion likewise consists of mesoblast and epiblast, but here the epiblast is external. The amnion proper

Fig. 272 and the following two wood-cuts are diagrammatic views of sections through the developing ovum, showing the formation of the membranes of the chick. (Foster and Balfour.)

A, B, C, D, E and F are vertical sections in the long axis of the embryo at different periods, showing the stages of development of the amnion and of the yolk sac.

I, II, III and IV are transverse sections at about the same stages of development.

i, i and iii give only the posterior part of the longitudinal section, to show three stages in the formation of the allantois.

e. Embryo. y. Yolk. pp. Pleuroperitoneal fissure. vt. Vitelline membrane.
af. Amniotic fold. al. Allantois.

is continuous with the skin of the embryo, and when the foetus is mature, the connection may be traced by the umbilical cord, round which it forms a sheath continuous with the skin at the umbilicus. This membranous sac enlarges, and in mammalia eventually becomes the large bag of liquid which contains the

fœtus. The amniotic liquid is of low specific gravity, consisting mainly of water containing traces of nitrogenous matter, phosphates and chlorides.

It contains albumin and some other nitrogenous constituents,

e. Embryo. a. Amnion. a'. Alimentary canal. vt. Vitelline membrane. af. Amniotic fold. ac. Amniotic cavity. y. Yolk. al. Allantois.

and a minute quantity of urea, which is thought to be derived from the foetal kidneys.

This fluid preserves the child from the effects of any jolts or jars caused by the movements of the mother, and similarly pro

tects the uterus of the mother by acting as a buffer between the foetus and the uterine wall. Before delivery it helps to dilate the os uteri, so that when the amnion is ruptured the head of the fœtus occupies the opening which has been gradually made by the fluid wedge. The outer part of the amniotic membrane, derived from the mesoblast, is of a tougher character than the inner epithelial layer, possesses muscular fibre and is capable of rhythmical contractions.

Diagrammatic sections of an embryo, showing the destiny of the yolk sac, ys. vt. Vitelline membrane. pp. Pleuroperitoneal cavity. ac. Cavity of the amnion. a. Amnion. a'. Alimentary canal. ys. Yolk sac.

(2) The Yolk sac is that part of the blastoderm which grows and envelops the yolk, which previously was only surrounded by the vitelline membrane. After the mesoblast has split into two layers, the splanchnopleure becomes bent inward at a point some distance from its origin, carrying with it the hypoblast. By this curve an upper constricted canal is differentiated from the large lower cavity. This upper canal becomes eventually the aliment