position and shape of the limb anlages are shown in Fig. 1, and the section demonstrates that they are formed by mesoderm, with a thin covering layer of ectoderm. The mesoderm is FIG. 6. Pig of 12.0 mm. No. 5. Transverse section 470. Ao.S., left dorsal aortic trunk; Au.d., right auricle of the heart; D.R., dorsal root of spinal nerve; D.C.S., left ductus Cuvieri; f., external groove between right and left auricles of the heart; G., ganglion of spinal nerve; L., anlage of fore-limb; Nch., notochord; Nv., nerve of limb; Oe., œsophagus; R.D., ramus dorsalis of spinal nerve; R.V., ramus ventralis of spinal nerve; S.a.c., septum of auricular canal; Scl. V., subclavian vein; Som., somatopleure ; Sp.c., spinal cord; S.s., septum superius; Tra., trachea; Val., atrio-ventricular valve; Ven.S., left ventricle; '.R., ventral root of spinal nerve. X 22 diams. very little differentiated, none of the skeletal elements being yet formed. The nerves and blood vessels are growing into the limb; the nerves are the rami ventrales (R. V.) of spinal nerves, and form branches within the limb; one of these branches, as yet unidentified, is shown at Nv. The vein Scl. V. is the so-called subclavian or axillary vein, a branch of the jugular, as explained below. The lower part of the section is occupied by the large heart lying in the pericardial chamber. The body-wall (Som.), or somatopleure, which forms the outer covering of this chamber, is quite thin, and without a trace of muscular or skeletal structures. The heart consists of two auricles and two ventricles. The auricles have thin walls and are separated from one another by a very thin membrane, the septum superius (S.s.). The right auricle (Au.d.) receives upon its dorsal side the opening of the vein or duct of Cuvier, this opening being guarded by valves; of these valves, the one towards the median line disappears, but the other, towards the right of the embryo, persists to form both the Eustachian and Thebesian valves of the adult. The corresponding opposite vein, or left duct of Cuvier (D.C.S.), is almost symmetrically placed, but does not have any communication with the left auricle, being instead connected at a lower level by a transverse venous trunk with the ductus Cuvieri of the right side. upper portion of the left duct (D.C.S.) is seen in the section to be somewhat constricted off from the lower portion, and in fact it shows the jugular vein of the head at its actual junction with the ductus. The ventricles of the heart are much larger than the auricles, and the left ventricle (Ven. S.) is already larger than the right; the external groove (f.), which marks the boundary between the two ventricles, is clearly shown by the section. The trabecular structure of the ventricles is well developed and affords a diagnostic mark by which the ventricles, however cut, may be easily recognized in sections. The development of the trabeculæ corresponds to the formation of the blood sinusoids of the heart, to which I have recently directed attention. The constricted region of the heart, which connects the auricles with the ventricles, is known as the auricular canal. A broad partition (S.a.c.) divides the canal into right and left channels, and at the ventricular ends of these channels the formation of atrio-ventricular valves The (Val.) is well advanced. The dorsal or trunk region of the section is formed chiefly by the more or less dense mesenchyma. It includes important structures. The spinal cord (Sp.c.) resembles that in Fig. 5, but is both larger and more differentiated. The fundamental morphological characteristics of a spinal nerve are well illustrated by the left nerve of Fig. 6. The dorsal root (D.R.) bears the ganglion (G.) and is joined a little below the level of the cord by the ventral root (V.R.) to form a single nerve trunk, which, however, soon subdivides into its two primary branches; the first or dorsal branch, ramus dorsalis (R.D.), bends at an acute angle upwards and outwards; the second or ventral continues downward and curves into the limb; owing to this curvature it must be traced through adjacent sections, but in Fig. 6 it can be seen again (R. V.) at the base of the limb, where it crosses the subclavian vein (Scl.V.). Some distance below the spinal cord is the small notochord (Nch.). Much lower, and between the two ducts of Cuvier, appear the œsophagus (Oe.) and trachea (Tra.), each a ring of entodermal epithelium with commencing condensation of surrounding mesenchyma. This condensed tissue is the anlage of the future submucosa and muscularis. Above the œsophagus, to the right and left, appears the dorsal aortic trunk, of which the left only (Ao.S.) is completely retained throughout life. It should be noted that the blood vessels, including the largest, have at this stage only endothelial walls, the adventitial and muscular coats being added later. All blood vessels are endothelial chambers, and this fundamental conception ought to be made clear to the student. We now pass to a section (No. 633) well below the heart, in order to study the characteristics of the Wolffian body, stomach, and liver. At this level, as comparison of the figures will show, the body of the embryo has its greatest dimensions. The upper edge (Um.) of the umbilical cord also appears in this section. The spinal cord, with the ganglia and nerves, presents essentially the same features as in Figs. 5 and 6. The notochord (Neh.) forms a small circle in section and is surrounded by an area over which the mesenchymal cells are more crowded or condensed than elsewhere. The condensed mesenchyma is the anlage of a vertebra (Vert.). It is more expanded laterally than dorso-ventrally. In the median line below the notochord is the large dorsal aorta (Ao.), which is No. 5. FIG. 7. Pig of 12.0 mm. formed by the union of the two dorsal aortic trunks of Fig. 6, embryo to the pelvis, where it forks to form the two allantoic arteries, which run to the umbilicus, and entering the umbilical cord supply the extra-embryonic or placental circulation. The aorta is surrounded by mesenchyma, and to this are, so to speak, appended the large Wolffian bodies (W.B.), one on each side. They are much larger, relatively to other parts, in the pig than in man or the rabbit. or the rabbit. The Wolffian body is the foetal or embryonic kidney, and is also termed the mesonephros. It consists of numerous epithelial tubules (Wt.), very much contorted, with blood spaces (sinusoids) between them, and of glomeruli, which always lie towards the median and inferior side of the organ. All of the tubules open into the single longitudinal canal, the Wolffian duct. This duct is always situated close to the surface on the ventral side of the organ, and is very easily distinguished in dissected embryos after the removal of the intestines and liver. In the figure it may be easily found in the left mesonephros (IV.B.), it being there the lowermost of the cavities drawn. On the median lower surface of the Wolffian body, underneath the glomeruli, is an accumulation of tissue (Gen.), the anlage of the genital gland. Below the aorta, on the right of the embryo, is the large ductus venosus, or upper end of the vena cava inferior, on its way past the right dorsal lobe of the liver towards the heart. Below the aorta on the left is the mesogastrium (Om.maj.), or future great omentum, by which the stomach is suspended from the median dorsal wall of the abdomen. The stomach (St.) is entirely upon the left side of the body, and is directly connected by means of the anlage of the lesser omentum (Om.min.) with the liver. The liver is by far the largest organ of the body; it takes up nearly half of the section. It is divided into four main lobes, two dorsal and two ventral, two on the right and two on the left. The reference line (Li.) runs to the left dorsal lobe. The liver consists of a complicated network of relatively large blood sinusoids, the spaces between which are occupied by the embryonic liver cells. Near the median line between the ventral lobes appears the gall-bladder (G.bl.), which is cut three times. The liver is attached in the median ventral line to the body-wall |