maxillary and nasal processes in the roof of the stomodeum the primitive palate (Fig. 49) is formed, and the olfactory pits extend backward above it. The posterior end of each pit is closed by an epithelial membrane, the bucco-nasal membrane, formed by the apposition of the nasal and stomodeal epithelium. By the rupture of these membranes the primitive choana or openings between the olfactory pits and the stomodeum are established. The floor of the nasal cavity is completed by the development of a pair of shelf-like palatine processes which extend medialward from the maxillary processes (Figs. 50 and 51); these coalesce with each other in the middle line, and constitute the entire palate, except a small part in front which is formed by the premaxillary bones. Two apertures persist for a time between the palatine processes and the premaxillæ and represent the permanent channels which in the lower animals connect the nose and mouth. The union of the parts which form the palate commences in front, the premaxillary and palatine processes joining in the eighth week, while the region of the future hard palate

FIG. 51.-Frontal section of nasal cavities of a human embryo 28 mm. long. (Kollmann.)

is completed by the ninth, and that of the soft palate by the eleventh week. By the completion of the palate the permanent choanæ are formed and are situated a considerable distance behind the primitive choanæ. The deformity known as cleft palate results from a non-union of the palatine processes, and that of harelip through a non-union of the maxillary, and globular processes (see page 199). The nasal cavity becomes divided by a vertical septum, which extends downward and backward from the medial nasal process and nasal laminæ, and unites below with the palatine processes. Into this septum a plate of cartilage extends from the under aspect of the ethmoid plate of the chodrocranium. The anterior part of this cartilaginous plate persists as the septal cartilage of the nose and the medial crus of the alar cartilage, but the posterior and upper parts are replaced by the vomer and perpendicular plate of the ethmoid. On either side of the nasal septum, at its lower and anterior part, the ectoderm is invaginated to form a blind pouch or diverticulum, which extends backward and upward into the nasal septum and. is supported by a curved plate of cartilage. These pouches form the rudiments of

the vomero-nasal organs of Jacobson, which open below, close to the junction of the premaxillary and maxillary bones.

The Limbs. The limbs begin to make their appearance in the third week as small elevations or buds at the side of the trunk (Fig. 52). Prolongations from the muscle- and cutis-plates of several primitive segments extend into each bud, and carry with them the anterior divisions of the corresponding spinal nerves. The nerves supplying the limbs indicate the number of primitive segments which contribute to their formation-the upper limb being derived from seven, viz., fourth cervical to second thoracic inclusive, and the lower limb from ten, viz., twelfth thoracic to fourth sacral inclusive. The axial part of the mesoderm of the limb-bud becomes condensed and converted into its cartilaginous skeleton, and by the ossification of this the bones of the limbs are formed. By the sixth week the three chief divisions of the limbs are marked off by furrows-the upper into arm, forearm, and hand; the lower into thigh, leg, and foot (Fig. 53). The limbs are at first directed backward nearly parallel to the long axis of the trunk,

and each presents two surfaces and two borders. Of the surfaces, one-the future flexor surface of the limb-is directed ventrally; the other, the extensor surface, dorsally; one border, the preaxial, looks forward toward the cephalic end of the embryo, and the other, the postaxial, backward toward the caudal end. The lateral epicondyle of the humerus, the radius, and the thumb lie along the preaxial border of the upper limb; and the medial epicondyle of the femur, the tibia, and the great toe along the corresponding border of the lower limb. The preaxial part is derived from the anterior segments, the postaxial from the posterior segments of the limbbud; and this explains, to a large extent, the innervation of the adult limb, the nerves of the more anterior segments being distributed along the preaxial (radial or tibial), and those of the more posterior along the postaxial (ulnar or fibular) border of the limb. The limbs next undergo a rotation or torsion through an angle of 90° around their long axes the rotation being effected almost entirely at the limb girdles. In the upper limb the rotation is outward and forward; in the lower limb, inward and backward. As a consequence of this rotation the preaxial (radial)

border of the fore-limb is directed lateralward, and the preaxial (tibial) border of the hind-limb is directed medialward; thus the flexor surface of the fore-limb is turned forward, and that of the hind-limb backward.

DEVELOPMENT OF THE BODY CAVITIES.

In the human embryo described by Peters the mesoderm outside the embryonic disk is split into two layers enclosing an extra-embryonic cœlom; there is no trace of an intra-embryonic coelom. At a later stage four cavities are formed within the embryo, viz., one on either side within the mesoderm of the pericardial area, and one in either lateral mass of the general mesoderm. All these are at first independent of each other and of the extra-embryonic celom, but later they become continuous. The two cavities in the general mesoderm unite on the ventral aspect of the gut and form the pleuro-peritoneal cavity, which becomes continuous with the remains of the extra-embryonic celom around the umbilicus; the two cavities in the pericardial area rapidly join to form a single pericardial cavity, and this from two lateral diverticula extend caudalward to open into the pleuro-peritoneal cavity (Fig. 54).

FIG. 54.-Figure obtained by combining several successive sections of a human embryo of about the fourth week (From Kollmann.) The upper arrow is in the pleuroperitoneal opening, the lower in the pleuropericardial.

Between the two latter diverticula is a mass of mesoderm containing the ducts of Cuvier, and this is continuous ventrally with the mesoderm in which the umbilical veins are passing to the sinus venosus. A septum of mesoderm thus extends across the body of the embryo. It is attached in front to the body-wall between the pericardium and umbilicus; behind to the body-wall at the level of the second cervical segment; laterally it is deficient where the pericardial and pleuro-peritoneal cavities communicate, while it is perforated in the middle line by the foregut. This partition is termed the septum transversum, and is at first a bulky plate of tissue. As development proceeds the dorsal end of the septum is carried gradually caudalward, and when it reaches the fifth cervical segment muscular tissue with the phrenic nerve grows into it. It continues to recede, however, until it reaches the position of the adult diaphragm on the bodies of the upper lumbar vertebræ. The liver buds grow into the septum transversum and undergo development there.

The lung buds meantime have grown out from the fore-gut, and project laterally into the forepart of the pleuro-peritoneal cavity; the developing stomach and liver are imbedded in the septum transversum; caudal to this the intestines project into the back part of the pleuro-peritoneal cavity (Fig. 55). Owing to the descent of

the dorsal end of the septum transversum the lung buds come to lie above the septum and thus pleural and peritoneal portions of the pleuro-peritoneal cavity (still, however, in free communication with one another) may be recognized; the pericardial cavity opens into the pleural part.

FIG. 55.-Upper part of celom of human embryo of 6.8 mm., seen from behind. (From model by Piper.)

The ultimate separation of the permanent cavities from one another is effected by the growth of a ridge of tissue on either side from the mesoderm surrounding

FIG. 56.-Diagram of transverse section through rabbit embryo. (After Keith.)

the duct of Cuvier (Figs. 54, 55). The front part of this ridge grows across and obliterates the pleuro-pericardial opening; the hinder part grows across the pleuroperitoneal opening.

With the continued growth of the lungs the pleural cavities are pushed forward

in the body-wall toward the ventral median line, thus separating the pericardium from the lateral thoracic walls (Fig. 53). The further development of the peritoneal cavity has been described with the development of the digestive tube (page 168 et seq.).

FIG. 57.-The thoracic aspect of the diaphragm of a newly born child in which the communication between the peritoneum and pleura has not been closed on the left side; the position of the opening is marked on the right side by the spinocostal hiatus. (After Keith.)

First Week. During this period the ovum is in the uterine tube. Having been fertilized in the upper part of the tube, it slowly passes down, undergoing segmentation, and reaches the uterus. Peters1 described a specimen, the age of which he reckoned as from three to four days. It was imbedded in the decidua on the posterior wall of the uterus and enveloped by a decidua capsularis, the central part of which, however, consisted merely of a layer of fibrin. The ovum was in the form of a sac, the outer wall of which consisted of a layer of trophoblast; inside this

was a thin layer of mesoderm composed of round, oval, and spindle-shaped cells. Numerous villous processes-some consisting of trophoblast only, others possessing a core of mesodermprojected from the surface of the ovum into the surrounding decidua. Inside this sac the rudiment of the embryo was found in the form of a patch of ectoderm, covered by a small but com