protuberant first or hyoid branchial arch (Hy.), and this arch is separated from the mandibular region by a groove (Au.) or ectodermal depression, which is later transformed into the meatus auditorius externus. In mammals there are four pairs of gillpouches, or so-called clefts, all of which are lateral diverticula of the pharynx. Owing to the curvature of the pharynx these diverticula are brought to different levels, and into different planes in the pig embryo at the stage we are considering; hence in a series of sections the various gill-pouches are encountered in different portions of the series, and thus it occurs that only one pouch, the third, is shown in the section. The student should clearly understand that the median region of the two-thirds gill-pouches in Fig. 5 is the pharynx proper, and that it gives rise to the anlage of the median thyroid, of which both the stalk (M.th.) and the glandular portion (Thyr.) appear. Just above the third gill-cleft may be seen the large, darkly stained lower ganglion (ganglion nodosum) of the vagus nerve (G.nod.), and just above the ganglion is situated the section of the jugular vein (Jug.). Close to the ganglion on its ectal side appear two fibrous nerve trunks, of which the one nearest the pharynx is the accessorius or eleventh nerve (N.x.i.), while the other nearest the jugular vein is the hypoglossus, which reappears (N.xii.) below the aortic arch. A little above the jugular vein is the section of the first cervical nerve (Cerv.1), laterad from which is the external branch (R.ex.xi.) of the spinal accessory nerve. This branch in the adult innervates the sternocleido-mastoid and trapezius muscles.

The lower part of Fig. 5 represents part of the head and shows the two nasal fossæ (Na.) closed towards the mouth side by the olfactory plate (Ol.pl.), an epithelial membrane somewhat similar to the closing plate of a gill-cleft. On the dorsal side of the olfactory fossæbelow in Fig. 5- the cerebral hemispheres are cut, their darkly stained wall bounding on each side the large lateral ventricle (L. V.).

The next section figured is No. 470, and is therefore much lower in the series. It was selected in order to show the anterior limb-buds, the ducts of Cuvier, and the heart. The

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 Sp.c.

D.R.

G.

Nch.

Scl.V.

R.V.

Tra.

D.C.S

Au.d.

Ven.S.

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; V.R., ventral root of spinal nerve. X 22 diams.

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. The 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

(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 (Oc.) 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 oesophagus, 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 (Nch.) 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

FIG. 7.-Pig of 12.0 mm. No. 5. Transverse section 633. Ao., aorta; D.V., ductus venosus ; Ec., ectoderm of the somatopleure; G., ganglion on dorsal root of spinal nerve; G.bl., gall bladder; Gen., anlage of genital gland; Li., liver; mes., mesenchyma of somatopleure ; msth., mesothelium of the somatopleure; Nch., notochord; N., spinal nerve; Om.maj., omentum major; Om.min., omentum minus; Som., somatopleure; St., stomach; Um., umbilical cord; Vert., anlage of the body of a vertebra; V.U.D., right umbilical vein; V.U.S., left umbilical vein; W.B., Wolffian body or mesonephros; W.t., Wolffian tubule. X 22 diams.

formed by the union of the two dorsal aortic trunks of Fig. 6, and which extends through the abdominal region of the