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of the embryo, but as the embryo rapidly extends, in all directions, from the margin of the orifice, the latter soon becomes relatively small. Ultimately the various parts of the margin of the orifice are approximated until they fuse together, closing the opening and forming a cicatrix on the ventral wall of the abdomen which is known as the umbilicus or navel.
Whilst the embryonic area is being folded into the form of the embryo, the neural groove on the surface of the area is being converted into the neural tube. After the neural tube is completely closed and separated from the surface, during the third week, the embryo is an elongated organism possessing a larger cephalic end, a smaller caudal end, attached by the body stalk to the chorion (Fig. 49), a continuous and unbroken dorsal surface, a ventral surface separated into cephalic and caudal portions by the umbilical orifice, two lateral surfaces right and left, and it contains within its interior three cavities: (1) The cavity of the neural tube, which becomes the cavities of the brain and the spinal medulla (Fig. 50); (2) the primitive alimentary canal, which is a portion of the entodermal vesicle constricted off during the folding of the embryonic area (Figs. 37, 40); (3) the embryonic cœlom. The cœlom consists of right and left portions which communicate at the margin of the umbilicus with the extra-embryonic cœlom, and with each other through the pericardial portion of the intra-embryonic cœlom in the ventral wall of the fore-gut of the embryo (Figs. 49, 90).
At this period the embryo is easily distinguished from the remainder of the zygote, and it is so far developed that indications of its general plan of organisation are discernible.
It has, as yet, no limbs, but the general contour of the head and body are defined. It possesses a notochord or primitive skeletal axis, afterwards replaced by the permanent vertebral column. On the dorsal aspect of the notochord lies the neural tube, which is the rudiment of the future brain and the spinal medulla. At the sides of the neural tube and the notochord are the mesodermal somites and the nerve ganglia (Figs. 40, 43).
Ventral to the notochord is the primitive alimentary canal (Fig. 50), closed at its cephalic end by the bucco-pharyngeal membrane, and at its caudal end by what was originally the caudal portion of the primitive streak, but which is now called the cloacal membrane because it separates the caudal end of the hind-gut, which becomes the entodermal cloaca, from the amniotic cavity (Fig. 50).
At the sides of the primitive alimentary canal are the right and left lateral parts of the cœlom, and between the dorsal angle of each half of the cœlom and the mesodermal somites of the same side lies the intermediate cell tract which is the rudiment of the greater part of the genito-urinary system (Figs. 39, 40).
Ventral to the fore-gut is the pericardial mesoderm, traversed by the pericardial portion of the cœlom, which is connected dorsally, on each side, with the corresponding lateral portions of the cœlom; and ventral to the hind-gut is the cloacal membrane. Between the pericardial region at the one end and the cloacal membrane at the other lies the umbilical orifice, through which the mid-gut communicates with the yolk sac, the intra-embryonic part of the cœlom with the extra-embryonic cœlom, and the allantoic diverticulum with the cloaca (Figs. 39, 50).
When it is first defined the embryo is entirely devoid of limbs (Fig. 51). During the third week a superficial ridge appears on each side, along the line of the intermediate cell tract in the interior. This is the Wolffian ridge, and upon it the rudiments of the fore and hind limbs, the limb buds, are formed, as secondary elevations; the fore-limb buds preceding the hind-limb buds in time. of appearance (Fig. 52).
Shortly after it has appeared, each limb bud assumes a semilunar outline; it projects at right angles from the surface of the body, and it possesses dorsal and ventral surfaces, and cephalic or preaxial, and caudal or postaxial borders. The
bud is the rudiment of the distal segment of the future limb, the hand in the case of the fore-limb, and the foot in the case of the hind-limb.
As the limb-rudiment increases in length the more proximal segments of the limb are differentiated, the forearm and arm in the case of the fore-limb, and the
leg and the thigh in the case of the hind-limb. At the same time the limbs are folded ventrally, so that their original ventral surfaces become medial and their original dorsal surfaces lateral, and the convexities of the elbows and knees are directed laterally. At a later period, on account of a rotation which takes place in opposite directions in the fore- as contrasted with the hindlimbs, the convexity of the elbow is turned towards the caudal end of the body and that of the knee towards the cephalic end. It is only at much later periods of development, as the erect posture is assumed, that the convexity of the elbow is directed dorsally and the convexity of the knee ventrally. The terminal or distal segment of each limb is, at first, a flat plate with a rounded margin, but it soon differentiates into a proximal or basal part and a more flattened marginal portion. It is along the line where these two parts are continuous that the rudiments of the digits appear. They become evident as small elevations on the dorsal surface of the limb bud about the fifth week; they extend peripherally, and by the sixth week the fingers project beyond the margins of the hand segment, but the toes do not attain to a corresponding stage of development until the early part of the seventh week.
FIG. 53. LATERAL VIEW OF A HUMAN EMBRYO-21 mm. greatest length, showing limb buds projecting from the Wolffian ridge.
(Keibel and Elze, Normaltafeln.)
The nails are later developments. They appear at the third month and reach the ends of the digits at the sixth month.
Each limb bud is essentially an extension of a definite number of segments of the body. It consists, at first, of a core of mesoderm covered by ectoderm.
grows the anterior branches of the spinal nerves of the corresponding segments are prolonged into it, together with a number of bloodvessels. The nerves remain as the nerves of the fully developed limb, but the blood-vessels are reduced in number and are modified until a smaller number of permanent main trunks is established.
The greater part, if not the whole, of the mesodermal core of the primitive limb-rudiment seems to be produced by the somatic mesoderm of the lateral plate. As the development proceeds it is differentiated into the cartilaginous, muscular, and other connective tissue elements which are the rudiments of the skeletal framework and the muscles and fascia of the adult limb.
It is not yet decided whether or not the muscle elements of the mesodermal core are derived from
the lateral plate mesoderm, or from FIG. 54.-LATERAL VIEW OF A HUMAN EMBRYO-9.5 mm. muscle cells which have migrated
into the limb, from the muscle Note that the limb rudiments no longer project at right angles from the side of the body but that they are bent ventrally. plates of the segments from which
the limb is formed and from which muscles of the body wall are developed; and although it is generally believed that the bone which replaces the cartilaginous skeletal rudiments is produced by mesodermal cells, it has been asserted that the bone-producing cells originate in the ectoderm and migrate from the surface into
THE EARLIER MODIFICATIONS OF THE PRIMITIVE ENTODERMAL ALIMENTARY CANAL AND THE FORMATION OF THE STOMATODEUM AND PROCTODÆUM.
The greater part of the permanent alimentary canal is derived from the entodermal sac and is therefore lined by entoderm cells. This part is enclosed in the embryo as the latter is folded off from the remainder of the zygote (Fig. 50), but the cephalic and caudal portions of the alimentary canal are formed by the enclosure of part of the external space and are, therefore, lined by ectoderm.
The cephalic part is a portion of a space called the stomatodæum which lies, at first, between the ventrally bent extremity of the head and the bulging pericardial region (Fig. 50). At a later period it is enclosed laterally by the rudiments of the maxillæ or upper jaws, and caudally by the mandibular rudiments.
When it first appears the stomatodæum is separated from the cephalic end of the entodermal portion of the primitive canal by the bucco-pharyngeal membrane, but when that septum disappears, during the third week, the stomatodæum communicates with the fore-gut. Subsequently, it is separated into nasal and oral portions, and the oral portion forms that part of the mouth in which the gums and teeth are developed.
The caudal part of the permanent canal is formed by the elevation of a surface
fold round a pit-like hollow called the proctodæum (Fig. 60), which is separated
from the caudal part of the entodermal portion of the alimentary canal, until about the fourth week, by a membrane called the Mesoderm of chorion anal membrane, a portion of the more extensive cloacal mem
Mesoderm of brane mentioned on p. 39.
Differentiation of the
Derivatives of the Lateral Wall-Shortly after the foregut is enclosed, and whilst it is still separated from the stomatodæum by the bucco-pharyngeal membrane, its cephalic extremity dilates to form the primitive pharynx and thereafter, a series of eight pouches are formed in its walls, five in each lateral wall; the pharyngeal or branchial pouches; two in its ventral wall, one near the cephalic extremity, the rudiment of the thyreoid gland, and a second situated more caudally, which is the germ of the respiratory system, that is, of the larynx, the trachea, the bronchi, and the epithelial lining of the lungs. The eighth pouch, Seessel's pouch, is formed in the dorsal wall, immediately caudal to the dorsal end of the bucco-pharyngeal membrane, and it projects into the floor of the primitive cranium.
FIG. 55.-SAGITTAL SECTION OF ZYGOTE SHOWN IN FIG. 38.
FIG. 56. SCHEMA showing the branchial pouches, the branchial clefts, the branchial bars, and the thyreoglossal duct and some of their derivatives. I., II., III., IV., and V., the five branchial bars.
Simultaneously with the formation of the pharyngeal pouches internally a series of clefts appear externally. They correspond in position with the first four pharyngeal pouches, and they are called the pharyngeal or branchial clefts.
By means of the pharyngeal pouches and clefts the lateral boundary of the cephalic part of the fore-gut, on each side, is divided into a series of bars, the pharyngeal or branchial bars, five in number, but the fifth is distinctly visible only in the inner aspect of the pharynx.
The first of the pharyngeal bars is the rudiment of the maxillary and mandibular regions. It is called the mandibular arch. The second is the hyoid arch, and the remainder are the branchial arches proper.
When they first appear, the arches extend from the level of the dorsal wall of the fore-gut to the pericardium but, as growth proceeds, and the neck is developed between the head and the pericardium, the ventral ends of the arches of opposite sides meet in the ventral wall of the primitive pharynx. The growth of the mandi
Ventral pancreas rudiment
FIG. 57.-SCHEMA OF A LONGITUDINAL SECTION OF AN EMBRYO. (After Mall, modified.) Showing dorsal and ventral divertricula for alimentary canal. The heart is not shown.
bular and the hyoid arches soon greatly exceeds that of the branchial arches proper, and the latter gradually recede from the surface until, on each side, they lie at the bottom of a depression, the precervical sinus, which is overlapped by the caudal border of the hyoid arch. As the overgrowth of the hyoid arch continues the opening of the precervical sinus to the surface is reduced to a narrow channel, the precervical duct. Afterwards this is obliterated, the sinus becomes the precervical vesicle,
FIG. 58. SCHEMA showing the formation of the precervical sinus, the branchial ducts, and the
but the position of the original aperture of the precervical duct is temporarily indicated by a sulcus, the precervical sulcus which soon disappears. The precervical vesicle lies at the side of the third pharyngeal cleft, and it is associated with the second and fourth clefts by narrow canals, the branchial ducts, which are the remains of the branchial clefts. Ultimately the precervical vesicle and the branchial ducts disappear, but it has been suggested that before the vesicle disappears a part of the lobe of the thymus of the same side is formed from its wall.