The two umbilical veins and the base of the umbilical cord. enter the liver directly from the cord. The right vein (V.U.D.) is already smaller than its left fellow (V.U.S.). They are connected respectively with the right and left ventral lobes of the liver. The right vein in a little later stage is no longer recognizable as an open channel. It will be noticed that the right and left sides of the abdominal cavity (splanchnocœle) are completely separated from one another, and that there is a special part of this cavity shown in the section between the stomach and the right dorsal lobe of the liver, and which is known as the lesser peritoneal space or cavity of the omentum. The body-wall (Som.), or somatopleure,1 consists of three layers, the ectoderm (Ec.), the mesenchyma (mes.), and the mesothelium (msth.). It is of the greatest importance for the student to understand the arrangement of the germ-layers in the somatopleure. The mesothelium (msth.) is commonly known in the descriptive anatomy of the adult as the peritoneal epithelium. In sections like that of Fig. 7 it can be followed not only over the inner surface of the body-wall but over the surface of the Wolffian body and liver, and upon the left side of the body also over the surfaces of the greater omentum, stomach, and lesser omentum. We see, in fact, that the body cavity is completely bounded by mesothelium, and that all the abdominal viscera are therefore outside of it. This conception, which is so important yet so difficult to the student of anatomy, is easily mastered by the study of the embryonic relations. From the sagittal series of the same stage many instructive pictures are obtainable. I have selected a median section of the head, and one passing through the principal cephalic ganglia, for engraving, and give here the latter. The magnification is 22 diameters, the same as for the transverse sections Figs. 2 to 7. To the student of anatomy such a section as is shown in Fig. 8 is highly instructive, for it exhibits in a single picture many important fundamental relations of the cephalic nerves, particularly of the second, fifth, seventh, 1 It is much to be regretted that German embryologists use the term "somatopleure" erroneously. eighth, ninth, and tenth. As the section is remote from the median plane, little of the brain appears, there being only a shaving from the lateral wall of the fore-brain (H.) and a section of the widest part of the hind-brain, which shows the cavity or lateral recess (R.L.) of the fourth ventricle. The auditory vesicle or otocyst (Ot.) is cut; its narrow upward prolongation is the anlage of the ductus endolymphaticus. The otocyst lies in a line with the great cephalic ganglion, occupying its invariable and permanent position behind the acustico-facial ganglion (Ac.F.) and in front of the glossopharyngeal. Only the lateral portion of the pharynx (Ph.) appears; this portion forms a wide diverticulum, almost slitlike (compare Fig. 4), from which extend the first and second entodermal gill-pouches. In the figure the pharynx has a small depression extending downward at the oesophageal end of the part shown in the section; this depression marks the beginning of the second cleft. Nothing is seen of the third and fourth clefts in the section, as these both lie nearer the median plane. The pocket or diverticulum of the cervical sinus (compare Fig. 1) appears (Cerv.S.) near the ganglion nodosum (G.nod.). It might be mistaken for a gill-cleft, but it is lined by ectoderm and its cavity can be easily traced through the series to the exterior. Cephalad from the sinus but close to it lies a small dark mass, the anlage of the thymus gland. The mass is produced by proliferation of the entodermal cells on the anterior side of the third cleft (compare Fig. 5, Thm.), and is penetrated by blood vessels which seem to be sinusoids, although their history has not been worked out. The jugular vein (Jug.), owing to its irregular course, is cut several times; its main stem (Jug.""") rises nearly vertically through the cervical region, and is, relatively to the size of the embryo, of huge diameter, and continues (Jug."") upward along the dorsal side of the vagus nerve to a level about halfway between the ganglion nodosum and ganglion jugulare, and as the vessel there curves inwards and forwards it is not encountered again until it bends outwards (Jug.) on its way past the trigeminal ganglion. A branch of the jugular vein (Jug.") is cut just above the ganglion. FIG. 8.-Pig of 12.0 mm. No. 7. Sagittal section 25. Aur., auricle of the heart; Ac.F., acustico facial ganglion complex; Co., colom around the heart or pericardial cavity; Cerv.S., diverticulum of the cervical sinus, just in front of which shows the anlage of the thymus, which is deeply stained; G.jug., ganglion jugulare of the vagus nerve; G.nod., ganglion nodosum of the vagus nerve; G.petr., ganglion petrosum of the glossopharyngeal nerve; G.tri., ganglion of the trigeminus nerve; H., lateral wall of the cerebral hemisphere; Jug., jugular vein; Jug', behind the trigeminus: Jug.", branch in front of the trigeminus; Jug.", main stem behind the vagus; Jug.", main stem descending to join the duct of Cuvier: m., an undetermined structure, probably the anlage of a lingual muscle; Md., mandible; N.V., root of the fifth or trigeminal nerve; N.op., optic nerve; N.xii., twelfth or hypoglossal nerve; Ot., otocyst; Ph., pharynx; R.L., recessus lateralis of the fourth ventricle; Ve., small branch of the jugular vein; Vent., ventricle of the heart. X 22 diams. The nerves are shown as follows: The optic nerve (N.op.) still has its central cavity, which, nearer the median plane, opens into the third ventricle of the brain, and in the section. resembles in shape an inverted U. On the side of the nerve towards the mouth there is a deep notch, the section of the choroid fissure. The trigeminal ganglion (G.tri.) is very large, and its trilobate form is clearly indicated by the figure. The lobe to which the reference line (G.tri.) runs gives off the ramus ophthalmicus; the lobe nearest the jugular gives off the ramus maxillaris inferior, while the middle lobe. gives off the ramus maxillaris superior. From the ganglion the fibres and nerve cells extend upwards to form the root (N.V.), which joins the hind-brain at a characteristic point,namely, at the summit of the Varolian bend and where the hind-brain is widest (compare Figs. 2 and 3). By its great size and by its topographical association with the lateral apex of the recessus lateralis of the fourth ventricle, the trigeminal ganglion may always be readily identified in sections of embryos. The acustico-facial ganglia (Ac.F.) may also be readily determined by their typical position immediately in front of the otocyst (Ot.). But it is quite difficult to identify the four components of this complex structure, namely: 1°, the motor root of the facial nerve; 2°, the facial or geniculate ganglion; 3°, the vestibular ganglion ; 4°, the cochlear ganglion. In Fig. 8 three divisions are shown. The large, darkly stained division, to which the reference line (Ac.F.) runs, and which lies nearest to the otocyst, is the vestibular portion of the acoustic ganglion; the small, light area occupying a middle position in the inferior part of the complex is the motor division of the seventh nerve, or lateral root of the facial; it can be followed to the brain, which it enters as four bundles of fibres; its path of entrance is shown better in frontal sections (Fig. 10). Just in front of the facial motor root lies a second smaller dark mass, the geniculate ganglion of the facial, with an upward prolongation, the sensory root. The ninth or glossopharyngeal nerve is represented by the ganglion petrosum (G.petr.) and its ascending sensory root. This nerve may be quickly identified because it is the first behind the otocyst. The upper ganglion of this nerve, the so-called Ehrenritter's ganglion, is represented by an accumulation of cells in the upper part of this root. As regards the tenth nerve, or vagus, both its ganglia and the fibrous trunk connecting them are shown. The upper or jugular ganglion (G.jug.) is nearly on a level with the otocyst, while the lower or nodosal (G.nod.) lies near the cervical sinus. To the nerve trunk Ec. A D.Z. between the two ganglia are G.tri. FIG. 9. Pig of 12.0 mm. Series No. 6. Frontal section 284. Ec., ectoderm; Ep., ependymal roof of the fourth ventricle; D.Z., upper portion of the dorsal zone of His; G.tri., ganglion trigemini; Jug., jugular vein; L.R., lateral root of the trigeminal nerve; mes., mesenchyma; T.S., tractus solitarius of W. His. X 22 diams. The frontal series has special value for the study of the hind- Jug. brain, its nerves, and of the otocyst. From this series I present here three figures of the head region. The first of these sections (Fig. 9) passes through the widest part of the hindbrain and through the roots of the trigeminus. The cavity of the hind-brain is enormously distended; it is bounded on the dorsal side only by a very thin layer of cells, the epithelial roof or ependyma (Ep.), which does not form part of the true nervous structures, although it passes into and is directly continuous with the dorsal zone (D.Z.) of the medulla oblongata, which is thus seen (also in Figs. 10, 11) to be only the thickened ventral wall of the neural tube, just as the ependyma is the attenuated dorsal wall. The trigeminal ganglion (G.tri.) is very large and sends its sensory fibres |