Figs. 113, 114 and 115 are different views of an embryo of this age. Fig. 113 shows the external appearance as seen from the right side; Fig. 114 is a longitudinal section to show the alimentary canal and parts in connec tion with it; while Fig. 115 is a diagrammatic representation of the prin cipal blood-vessels in situ.
The embryo is bent on itself as shown in the figures, and is closely in vested by the amnion; it is connected with the yolk-sac by a short but narrow stalk, and with the chorion by the wide allantoic stalk, now some what longer than it was before.
The body measures along its greatest diameter .27 inches, but if straightened out would be about .54 inches in length. In the head the several divisions of the brain are clearly visible, as are also the visceral arches and clefts, and the eye and ear. In the body the full number of protovertebrn, thirty-five, is present; and the limbs are still short and very broad buds, whose bases extend over several segments. Immediately underneath the neck is seen the large prominence formed by the heart, and, below this again, a lesser one due to the liver.
The alimentary canal forms a continuous and but slightly twisted tube.
The mouth, on the under surface of the head, opens into the buccal cavity, from which the pituitary diverticulum is given off towards the base of the mid-brain, while in the floor of the hinder part is the rudi mentary tongue. Behind this the buccal cavity passes into the pharynx, which opens to the exterior by two visceral clefts on each side. In the floor of the pharynx is a slit-like opening, the glottis, leading into a short canal, the trachea, which bifurcates into two blind sacs, the rudiments of the lungs.
Behind the pharynx the alimentary canal is continued as a narrow tube, the oesophagus, with the lungs on either side of it. The oesophagus opens into a stomach which is but very slightly dilated, and is continued into the small intestine. This runs forward and (Fig. 114) joins the large intestine at a rather sharp angle. From this angle the cavity of the yolk-stalk, leading to the yolk-sac, arises. The large intestine is at first tubular, but after giving off the cavity of the allantois it expands to form a eloacal sac, which opens to the exterior at the anus: just before doing so it receives, on its dorsal surface, the openings of the Wolffian ducts, the ducts of the primitive kidneys. A slight dilatation near the commencement of the allantoic stalk is the rudiment of the future urinary bladder.
The liver is a large organ whose position has been already noticed; it opens by a short bile-duct into the intestine just below the stomach.
The vascular system, which is shown in Fig. 115, has attained very considerable complexity. The heart, which is of great size, lies between the head and the liver, and already exhibits all the principal divisions of the adult. It is bent on itself like a letter 0, whereof the first or upper loop is the auricular portion of the heart; the second or lower loop, which is in very close proximity to the cerebral hemisphere, is the ventricular part; and the terminal limb of the (./) is the aortic bulb which runs back wards (upwards in the figure) beneath the lower wall of the head. The auricular portion of the heart is very wide transversely; it is partially divided by a semilunar fold into right and left auricles, whereof the right auricle receives (1) venous blood brought from the body of the embryo by two large veins, the right and left ductuR cuvieri, each of which is formed by the union of an anterior cardinal or jugular vein returning blood from the head, and a posterior cardinal vein coming from the hinder part of the body and chiefly from the Wolffian bodies or primitive kidneys; and (2) blood which is more arterial in character, brought back from the chorion by the umbilical or allantoic veins, and discharged into the right auricle by the vena cave inferior. The opening of the vena cava inferior into the right auricle is guarded by two prominent lips, the outer one of which is the Eustachian valve, which direct the blood from the inferior cave into the left auricle, which also receives the exceedingly small pulmonary. veins.
The ventricular portion is partially divided into right and left ventricles by a septum incomplete above. The aortic bulb contains at present only a single tube, the cardiac aorta, into which both ventricles discharge; this runs back (up in the figufe) to the hinder part of the floor of the buccal cavity, and then gives off on each side a series of aortic arches which run up in the visceral arches, forming the side walls of the pharynx, and unite together above the pharynx to form the dorsal aorta, which runs back to the hinder end of the body. Of these aortic arches there are at first five on each side. The first and second aortic arches, lying in the mandibular and hyoidean arches respectively (Figs. 113 and 115), have al ready lost their primitive connections with the dorsal aorta, and together form the external carotid artery, the first arch representing its lingual branch.