Histology and Embryology 1941 - Embryology 2

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Nonidez JF. Histology and Embryology. (1941) Oxford University Press, London.

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   Histology and Embryology 1941: Histology - 1 The Cell | 2 The Tissues | 3 The Organs     Embryology - 1 General Development | 2 Organogenesis | Bibliography

Part Two - Organogenesis

The organs of the vertebrate body arise from the three germ layers. The endoderm gives rise to the digestive and respiratory systems. The mesoderm contributes the supporting tissues, the vascular, urogenital and muscular systems, as well as the body cavities. The ectoderm is the source of the integument, central and peripheral nervous systems, and the sense organs.

Endodermic Derivatives

The Digestive System

I. Introductory Remarks

A. Ectodermic contributions.

1. Stomodaeum. In the early embryo the fore-gut ends blindly; its endoderm is fused with the ectoderm to form the oral (pharyngeal) membrane which is the floor of an external depression, the stomodaeum. The latter develops into the front part of the mouth and gives rise to the enamel of the teeth, salivary glands and mucosa of the nose and palate, which are, therefore, ectodermic. The oral membrane ruptures at the beginning of the 5th week. 2 . Proctodaeum. The hind-gut becomes the cloaca which later is divided into the rectum and the urogenital sinus. The cloacal membrane (endoderm pips ectoderm) ruptures at the end of the 7th week. After this a short ectodermic proctodaeum is added to the rectum as its anal canal.

B. Relation to the primitive gut. The fore-gut gives rise to the posterior part of the mouth cavity, pharynx, oesophagus, stomach and a good part of the small intestine. The hind-gut forms the rest of the small intestine, colon and rectum. The intermediate region (midgut) is unimportant in man.

C. Pharyngeal derivatives. The endodermic lining of the pharynx gives rise to organs which in the adult have no connection with the digestive tract (thyroid, parathyroids and thymus).

II. The Organs of the Mouth Cavity

Since the tongue develops from the branchial arches it will be considered in the next section.

A. The teeth. They are the homologues of the scales of the elasmobranch fishes (i.e. products of the skin) and as such arise from two different sources: the epidermis, which forms the enamel, and a dermal papilla which is transformed into dentine and tooth pulp (dental papilla).

1. The dental lamina. This is a slightly curved epithelial ridge which sinks into the substance of the primitive gum.

2. Enamel organs. They are thickenings which develop at intervals along the lamina. Early in the 3rd month the deeper side of each organ presses against the dense accumulation of mesenchyme of the dental papilla.

a. Number. Ten enamel organs develop in each jaw. They are the primordia of the deciduous (milk) teeth.

b. Structure. The enamel organ or sac resembles an inverted cup with its concavity applied against the dental papilla.

(1) Outer enamel cells. They line the convex portion of the cup. At first cuboidal they later become flat. They do not contribute to the formation of the tooth.

(2) Inner enamel cells (ameloblasts). They line the concavity of the sac. They are columnar and secrete the enamel which covers the crown of the tooth.

(3) Enamel. Is laid down as a fibrillar layer which then calcifies in the form of elongated prisms, one for each ameloblast. It appears first at the apex of the crown and extends gradually toward the region of the future root, which it does not invest.

(4) Enamel pulp. This is derived from the epithelial elements between the outer and inner enamel cells. They are transformed into a reticulum.

c. Nasmyth's membrane (dental cuticle). Represents the remains of the enamel organ covering the apex of the tooth at eruption. It soon wears off.

3. The dental papilla. This is the mesodermic or dermal portion of the tooth.

a. Odontoblasts. The superficial cells (facing the ameloblasts) become the columnar odontoblasts which secrete the dentine (p- 55 ) b. Dental pulp. The remaining mesenchyme differentiates into the dental pulp.

4. The dental sac. This is formed by the mesenchyme which surrounds the developing tooth. The inner portion produces a layer of osteoblasts at the level of the root; they deposit the cementum. When the tooth fills its alveolus the sac becomes the peridental membrane (p. 55).

5. Disintegration of the dental lamina. It occurs after the first set of enamel organs has been laid down, but its free edge gives rise to the enamel organs of the permanent teeth. A backward growth of the lamina produces the enamel organs of three molars not represented in the primary dentition.

6. Eruption. This is caused by growth of the root; the crown pushes to the surface and compresses the gum, which atrophies at this point.

7. Permanent teeth. They develop in the same way as the deciduous and lie on the lingual side of the latter. By their rapid growth at certain periods (6th to 18th year, according to the tooth) they press against the deciduous teeth, the roots of which undergo partial resorption. This causes their shedding.

B. The palate. Its development begins with the formation of the :

1. Lateral palatine processes. They are shelf-like folds of the maxillae which project toward the midline.

a. Fusion. They fuse with each other and with the greater portion of the nasal septum; the fusion begins anteriorly.

b. Formation of the hard palate. This is formed of bone which arises in the anterior part of the fused processes.

c. Soft palate. The caudal part of the processes does not unite with the nasal septum and is not ossified; this is the soft palate and its free posterior apex, the uvula, notched at first.

2. Cleft palate. Results from total or partial failure of the fusion of the lateral palatine processes. It may be associated with hare-lip (p. 1 12).

III. The Pharynx

The lateral walls of the embryonic pharynx form five pairs of outpocketings (pharyngeal or branchial pouches), the last of which is rudimentary in man. They come into contact with the ectoderm of corresponding branchial grooves and fuse with it, forming the closing plates, which become perforated in human embryos only occasionally.

A. The branchial arches. Their position has already been described (p. hi). They give rise to several head structures (jaws, face and external ear) and various muscles, cartilages and bones. On the floor of the pharynx they contribute to the formation of the tongue and epiglottis.

1. The tongue. The body or apical half of the organ arises in front of the second branchial arches, the root develops primarily from the second arches, but receives additions from the third and fourth. The boundary line between the body and root is the Vshaped sulcus terminalis.

a. The body. It arises from three primordial

(1) Tuberculum impar, or median primordium present in the pharyngeal floor between the first pair of pouches. It contributes little or nothing to the formation of the human tongue, according to some authors.

(2) Paired lateral swellings, located in the mandibular arches; they meet at the median septum linguae.

b. The root. Arises from a median primordium (copula) which is produced by the union of the second branchial arches in the midline. The adjacent portions of the arches join the copula.

c. Foramen caecum. Between the tuberculum impar and the copula is the point of origin of the thyroid diverticulum, represented by a pit (foramen caecum) in the adult.

d. Musculature. Arises from mesoderm of the floor of the mouth.

2. The epiglottis. The copula connects with a rounded prominence developed from the bases of the third and fourth branchial arches : this is the epiglottis, which becomes concave on its ventral (laryngeal) surface.

B. The pharyngeal (branchial) pouches and their derivatives. The first and second pharyngeal pouches open into broad lateral expansions of the pharynx, while the third and fourth communicate with the pharyngeal cavity through narrow canals. The different pouches give rise to a number of structures:

1. The auditory (Eustachian) tube and tympanic cavity. These arise from the first pair of pouches. Each tube is formed by a dorsal outpocketing or wing of the pouch and opens into the expanded tympanic cavity. The first branchial groove deepens as the external auditory meatus, while its closing plate becomes the ear-drum (tympanic membrane).

2. The palatine tonsils. The dorsal angle of the second pouch persists as the tonsillar fossa, which gives rise to the crypts of the palatine tonsils. (The pharyngeal and lingual tonsils are not pharyngeal pouch derivatives.)

3. The thymus. It appears as two large ventral diverticuli of the third pair of pharyngeal pouches. The corresponding diverticuli of the fourth pouches have been regarded as rudimentary thymic primordia which usually atrophy.

a. Loss of the lumina. The diverticuli become solid epithelial masses and lose their connection with the wall of the pouch.

b. Fusion. The fusion of the epithelial masses in the midline is superficial and produces the body of the gland, which gradually takes its permanent position in the thorax.

c. Formation of the reticulum. The epithelium is changed into a reticular framework through formation of large cytoplasmic vacuoles. The Hassal bodies are supposed to arise from this endodermic reticulum.

d. Invasion by lymphocytes. Takes place toward the end of the 3rd month when the organ begins its differentiation into cortex and medulla.

4. The parathyroids. They arise from the dorsal diverticuli of the third and fourth pharyngeal pouches and, accordingly, are designated as parathyroid III and IV, respectively.

a. Migration. They leave the pouches in the 7th week and migrate caudad.

b. Permanent position. Parathyroids III are dragged downward by the thymic primordia so that they come to lie at the caudal thyroid border, while parathyroids IV are nearer the cranial border.

5. The ultimobranchial bodies. Usually regarded as derived from the fifth pouches, they leave their site of origin and migrate caudad with parathyroids IV, fusing with the thyroid. Their ultimate evolution varies according to the species; in man it is claimed that they give rise to thyroid tissue.

C. The thyroid gland. It develops as a diverticulum arising from the floor of the pharynx.

1. Thyro-glossal duct. The thyroid diverticulum is connected with the pharyngeal epithelium through this duct which, if persistent, opens in the foramen caecum of the tongue. It usually atrophies during the 6th week.

2. Loss of the lumen. The body of the diverticulum becomes bilobed and through loss of its lumen is converted into a solid structure composed of epithelial plates.

3. Formation of follicles. Cavities representing the follicles begin to appear in the epithelial plates; they soon acquire colloid. This process ends by the end of the 4th month.

IV. The Digestive Tube

The development of its different regions is rather uniform except for such differences as size, shape and position. The epithelial lining is endoderm invested by splanchnic mesoderm; the latter gives rise to the other layers.

A. Oesophagus. Its development is characterized by a gradual differentiation of the walls.

B. Stomach. In early embryos it is a spindle-shaped dilatation of the gut.

1. Formation of the curvatures. The dorsal border grows faster than the ventral and this unequal growth causes the formation of the greater curvature. The fundus arises as a sacculation near the cardia.

2. Mesenteries. The dorsal mesogastrium grows faster than the ventral; it forms the omental bursa (p. 129).

3. Rotation. The stomach rotates about its long axis until the greater curvature lies on the left and the lesser (primitive ventral border) is on the right.

C. Intestine. In early embryos the intestine forms a single loop which enters the coelomic extension of the umbilical cord. Later the caudal limb of the loop develops a swelling which indicates the caecum, and the loop is withdrawn into the body cavity.

1. Torsion. This takes place about the superior mesenteric artery in such a way that the cranial limb of the loop is carried to the right and caudad of the caudal limb; the latter shifts to the left and cephalad.

2. Elongation and formation of loops. Rapid growth of the small intestine causes the formation of its characteristic loops. The first of these is the duodenum.

3. Differentiation of the colon. The formation of the three portions of the colon is a complicated process and will not be described here.

V. The Liver

It first appears as the hepatic diverticulum which is an outpocketing of the ventral floor of the fore-gut.

A. Penetration into the ventral mesentery. Ventral growth of the hepatic diverticulum causes its penetration into the splanchnic mesoderm of the ventral mesentery; the latter is split into halves which encapsulate the liver, forming the capsule of Glisson.

B. Formation of the hepatic cords. Soon after the formation of the diverticulum its blind end produces solid anastomosing cellular cords which constitute the parenchyma of the liver.

C. Formation of the sinusoids. The hepatic diverticulum lies between the vitelline (ophalomesenteric) veins, which form plexuses in the ventral mesentery. The epithelial cords grow between these venous plexuses which become the sinusoids.

D. Ducts. The hepatic duct and the common bile duct (ductus choledochus) are the stem portions of the original hollow hepatic diverticulum, while the gall bladder and cystic duct represent a secondary, more caudally placed outpocketing.

VI. The Pancreas

The pancreas arises from two primordia (dorsal and ventral), which are outpocketings of the endoderm lining the duodenum.

A. Dorsal pancreas. It extends into the mesentery as a solid cell mass connected with the duodenum by a duct.

B. Ventral pancreas. This remains smaller and its short duct is dragged away from the duodenum by the common bile duct, from which it secondarily arises.

C. Fusion of the primordia. The primordium of the ventral pancreas is shifted to the dorsal mesentery near the dorsal pancreas with which it fuses completely. It forms the head of the organ.

D. Ducts. The distal segment of the dorsal duct fused with the entire ventral duct form the main pancreatic duct (of Wirsung). The proximal segment of the dorsal duct becomes the accessory duct (of Santorini) .

The Respiratory System

The respiratory system (except the nasal passages) arises as an outpocketing or evagination of the ventral wall of the fore-gut

I. Early Development

A. The laryngo-tracheal groove. It appears in very early embryos along the floor of the fore-gut, caudal to the pharyngeal pouches. It becomes the larynx and trachea.

B. Lung buds. The rounded, posterior end of the groove projects ventrally and represents a single primordium of the lungs. It splits caudally into two outpocketings, the lung buds, which remain connected with the future trachea.

C. Tracheo-oesophageal grooves. They form on the lateral aspects of the fore-gut; their deepening toward the midline and subsequent fusion cause the separation of the trachea from the oesophagus.

II. The Larynx

A. Arytenoid swellings. They bound laterally the upper end of the laryngeal portion of the laryngo-tracheal groove.

1. Fusion with the epiglottis. Through fusion with the epiglottis the arytenoid swellings produce a U-shaped ridge, the furcula. The fusion is temporary, however.

2 . Bending. The parallel swellings are bent at the middle so that their cranial positions diverge laterally, nearly at right angles to their caudal portions.

3 . Formation of the glottis. When the arytenoid swellings lose contact with the epiglottis the entrance to the larynx — previously T-shaped and obliterated through fusion of the epithelial lining — becomes oval and patent.

B. Development of the laryngeal cartilages and muscles. They arise from condensations of mesenchyme derived from the fourth and fifth pairs of branchial arches (p. 150) .

III. The Trachea

Its development is mainly represented by elongation and the differentiation of its walls.

IV. The Lungs

The right lung bud soon becomes larger and is directed caudad.

A. Bronchial buds. The right lung bud gives off two lateral bronchial buds, the left only one.

1. Relation to the lobes. The bronchial buds indicate the position of the upper and middle lobes on the right side, the upper lobe on the left. The lower lobes arise from the blind ends of the lung buds.

2 . Eparterial bronchus. This is the apical bronchus of the right upper lobe, so called because it alone passes dorsal to the pulmonary artery.

3 . Cardiac bronchus. This is the ventral bronchus of the right lower lobe, which in a way compensates for the loss of a corresponding branch of the left side, eliminated so as to make room for the heart.

4. Branching of the buds. The bronchial buds branch repeatedly, and their epithelium becomes lower; in the terminal portions (pulmonary alveoli) it is actually flattened. The existence of alveolar epithelium, however, is questioned by some (p. 69).

B. Development of the lobes. The respiratory tree develops in a median mass of mesenchyme which resembles a broad mesentery and is later called the mediastinum.

1. Invasion of the pleural cavities. The developing lungs, invested by a layer of mesoderm, grow out laterally into the pleural cavities. The branching of the bronchial buds takes place within this mesoderm, and the external lobation becomes apparent.

2. Differentiation of the mesoderm. The mesenchyme surrounding the bronchial tree produces the tissues of the wall of the bronchi.

3. Visceral and parietal pleura. The surface of each developing lung is covered with mesoderm lined externally by mesothelium; this is the visceral pleura. The corresponding layer lining the thoracic wall is the parietal pleura.

C. The lungs at birth. Until birth the lungs are small and compact and do not fill the pleural cavities. With the onset of breathing they gradually distend with air and the lung tissue becomes light and spongy.

Mesodermic Derivatives

The Mesenteries And Coelom

I. The Mesenteries

Soon after it is formed the primitive gut is enclosed into a mesentery, which arises through fusion of the splanchnic mesoderm of the two sides in the midline. The gut subdivides this primitive mesentery into dorsal and ventral halves.

A. The dorsal mesentery. The pharynx and upper oesophagus have no mesentery; the lower oesophagus, like the trachea, lies in the future mediastinum. The rest of the digestive tract is suspended from the dorsal body wall by a continuous mesentery.

1. Regional names. The portion which attaches the stomach to the dorsal body wall is the dorsal mesogastrium or greater omentum; then there is a mesoduodenum, mesentery of the small intestine, mesocolon and mesorectum.

2. The formation of the omental bursa. The lengthening and bending of the dorsal mesogastrium toward the left during rotation of the stomach (p. 126) forms the omental bursa. In young embryos (up to 10 mm.) the bursa is bounded mesially by the dorsal mesogastrium (greater omentum) and the right wall of the stomach, laterally by the right lobe of the liver and the mesentery in which the hepatic portion of the inferior vena cava develops (caval mesentery).

a. Epiploic foramen (of Winslow). The bursa communicates to the right with the vestibule; the latter opens into the peritoneal cavity through this foramen situated between the liver and the caval mesentery.

b. The inferior recess. This is due to enlargement of the bursa to the left and caudad. Posteriorly it ends blindly.

c. Fusion with the dorsal body wall and colon. The dorsal wall of the bursa fuses with the dorsal body wall as well as with the colon and its mesentery (mesocolon).

d. Obliteration of the inferior recess. Its anterior and posterior walls fuse. In the adult it is reduced to a space between the stomach and dorsal fold of the greater omentum, the latter being largely fused with the dorsal body wall.

3. Secondary fusions of the dorsal mesentery. They occur as the result of the upright position in man and the higher apes. The most important leads to the formation of the transverse mesocolon and fixes the duodenum and pancreas to the dorsal body wall.

B. The ventral mesentery. It is associated intimately with the development of the heart and liver. The portion between the liver, stomach and duodenum is the lesser omentum. The greater part of the ventral mesentery disappears early and the right and left peritoneal cavities merge into a single cavity. What remains gives rise to the falciform and coronary ligaments of the liver.

II. The Coelom (Body Cavity)

In early embryos the two halves of the coelom merge into a single cavity in front as well as ventral to the heart, but caudal to the latter the two coelomic cavities remain independent. The coelom can be compared with an inverted U; the bend is occupied by the pericardial cavity, while the limbs represent the pleuro-peritoneal canals. A. Division into separate cavities. The separation of the pericardium, pleural cavities and peritoneal cavity is effected by the development of three sets of partitions.

1. The septum transversum. This is located caudal to the heart and fills the space between the gut, yolk stalk, and ventral body wall, separating the pericardial and peritoneal cavities.

a. Pleuro-pericardial canals. Since the septum does not extend dorsal to the gut it leaves on each side a canal through which the pericardial and peritoneal cavities communicate.

b. Migration. The septum, at first in the cervical region, undergoes a gradual displacement caudad. The permanent location is reached in the two-month embryo.

2. Pleuro-pericardial membranes. They separate the pleural cavities from the pericardial cavity; they develop around the common cardinal vein of each side.

3. Pleuro-peritoneal membranes. These gradually separate the pleural cavities and the single peritoneal cavity.

B. The diaphragm. The partitioning of the coelom results in the formation of the diaphragm.

1. Origin. The diaphragm of the adult is derived from four sources :

a. Its ventral portion from the septum transversum.

b. Its lateral parts from the pleuro-peritoneal membranes, and:

c. Derivatives from the body wall.

d. The median dorsal portion is contributed by the dorsal mesentery.

2. Diaphragmatic hernia. Since the diaphragm arises from multiple sources, imperfect development or absence of one of them leads to this defect, which is more common on the left side due to failure of the formation of the pleuro-peritoneal membrane.

The Vascular System

I. Origin of the Blood and Hemopoiesis in the Embryo

The blood and the blood vessels first appear in the splanchnic mesoderm that invests the yolk sac.

A. Blood islands. These are solid masses of cells which are soon changed into vesicles.

1. Formation of the endothelium. The peripheral cells of the blood islands are arranged into a single layer of flattened cells, which may also arise from the surrounding mesenchyme.

2. Blood cells. The other cells in the island become erythrocytes.

3. Plasma. It accumulates within the island and separates the blood cells, which thus float in it.

B. Area vasculosa. This is a network of primitive vessels in the wall of the yolk sac. It arises through fusion of the blood islands.

C. Embryonic vessels. The first vessels to appear within the embryo proper arise as clefts within the body mesenchyme.

D. Sites of hemopoiesis. The formation of blood takes place in the following locations:

1. The yolk sac (4th week).

2 . Body mesenchyme and blood vessels (5th week) .

3 . Liver sinusoids (6th week).

4. Spleen, lymph nodes and thymus (2nd to 3rd months).

5. Bone marrow, from 3rd month on throughout postnatal life. (For hemopoiesis in postnatal life see p. 10.)

II. The Early Vascular System

The embryonic vessels arise through coalescence of the vesicles in which the blood develops. The first paired vessels to appear are the:

A. Aortae. They run anteriorly under the fore-gut (ventral aortae) and bend dorsally in front of its blind end to become the dorsal aortae. The latter soon fuse into a single descending aorta.

B. Cardiac tubes. The short ventral aortae are connected posteriorly with the cardiac tubes, which later fuse into a single heart.

C. Umbilical arteries. The dorsal aorta give off caudally these two vessels which enter the body stalk on their way to the chorion.

D. Umbilical veins. These course in the body wall and return the blood from the chorion to the heart.

E. Vitelline vessels. They are: a pair of vitelline arteries arising from the dorsal aorta and ending in the area vasculosa of the yolk sac, and a pair of vitelline veins opening into the heart.

F. Embryonic veins. They arise within the body of the embryo.

1. Anterior cardinal (precardinal) veins. They drain the blood from the head region; they course in the somatopleura.

2 . Posterior cardinal (postcardinal) veins. They return the blood from the posterior end of the body.

3 . Common cardinals (ducts of Cuvier). Before entering the heart the two cardinals of each side form this common trunk which crosses the pleuro-peritoneal canal.

G. Aortic arches. They connect the ventral with the dorsal aortae.

The first pair is the anterior bend of the ventral aorta as it becomes dorsal; four more pairs develop more caudally (p. 136 ).

III. Development of the Heart

The heart arises through fusion of paired primordia (cardiac tubes) just posterior to the ventral aortae.

A. Early development.

1. Fusion into a single tube. This is caused by the process of folding which gives rise to the fore-gut. The single cardiac tube has an endothelial lining.

a. Dorsal mesocardium. The cardiac tube is suspended from the dorsal body wall by this double sheet, formed by fusion of the two plates of splanchnic mesoderm in the midline. It soon disappears.

b. Ventral mesocardium. This develops in the chick embryo but it is absent in the mammal due to the precocious splitting of the mesoderm.

c. Epi-myocardium. The layer of thickened splanchnic mesoderm that surrounds the endothelial tube and gives rise to the epicardium and myocardium.

2. Division into regions. The single cardiac tube soon shows the following regions :

a. The sinus venosus, which receives the blood from the umbilical, vitelline and common cardinal veins; it develops a pair of valves which guard the opening into:

b. The atrium, placed anteriorly to the sinus and communicating with:

c. The ventricle through a narrow atrio-ventricular canal.

d. The bulbus, continuous with the short ventral aortae.

B. External changes. They result from the bending of the single cardiac tube, which grows in length faster than the cavity in which it is contained.

1. Bulbo- ventricular loop. This chief early flexure is to the right, and it has the shape of a U; one limb is the bulbus, the other the ventricle.

2. Formation of. the atria. Due to growth of the bulbo-ventricular loop the atrium and sinus venosus shift cephalad. The single atrium forms lateral outpocketings which become the paired atria; the furrow between them is the interatrial sulcus.

3. Formation of the primitive ventricle. With continued growth of the bulbo-ventricular loop its two limbs become confluent: the single chamber is the primitive ventricle, separated from the atria by the deep coronary sulcus.

4 . Interventricular sulcus. This is the external manifestation of the formation of the interventricular septum, which separates the two ventricles.

C. Internal changes. They lead to the formation of the fourchambered heart characteristic of birds and mammals.

1. Development of the atria. The partitioning of the atria is a gradual process which is not completed until after birth.

a. Septum primum. At first it is a sickle-shaped partition that grows from the mid-dorsal atrial wall: it advances toward the ventricle and its free edge fuses with the endocardial cushions (p. 136), which have split the primitive atrio-ventricular canal into right and left halves.

(1) Foramen interatriale primum. This is the space enclosed within the concavity of the septum: before the latter finally reaches the endocardial cushions a secondary perforation occurs, the:

(2) Foramen interatriale secundum (ovale I), which is located near the attachment of the septum to the dorsal atrial wall.

b. Septum secundum. It makes its appearance just to the right of the septum primum. It arises from the caudal end of the left valve of the sinus venosus (p. 135). It is also sickle-shaped; its concavity is the:

(1) Foramen ovale (ovale II), which never disappears as such and becomes the oval fossa of the adult heart.

(2) Relation of the foramen ovale II to the septum primum. Since the foramen ovale II is placed more ventrally than the interatriale secundum (ovale I), it is overlapped by the imperforated portion of the septum primum.

(3) Passage of the blood through the foramen ovale. The portion of the septum primum covering the foramen ovale II serves as a flap-like valve permitting passage of the blood from the right to the left atrium, but not in the reverse direction.

c. The atrial septum. It arises after birth through fusion of the edges of the septum secundum with the septum primum. The edge of the former becomes the limbus of the oval fossa, the septum primum the membranous portion of the fossa. This fusion closes the foramen.

2. The sinus venosus. The sinus venosus soon develops a large right and a smaller left horn. The horns receive the blood returning to the heart through the primitive embryonic veins (p. 132).

a. The right horn. After their formation (p. 139), the superior and inferior vena cava open into the right horn. Rapid atrial growth incorporates the horn into the wall of the right atrium, and the venae cavae open directly into the latter.

b. The left horn. It becomes the coronary sinus.

c. Transformations of the valves. The opening of the sinus venosus into the atrium is guarded by two valves (valvulae venosae). The left valve is incorporated into the septum. The right forms the:

(1) Crista terminalis, which is a continuation of its cephalic portion: and the:

(2) Eustachian and Thebesian valves, which arise from the remainder of the valve. The former (valve of the inferior vena cava) is larger than the Thebesian (valve of the coronary sinus).

3. The pulmonary veins. The single pulmonary vein of the early embryo splits into right and left branches which in turn bifurcate. During the rapid growth of the atria first the single stem, then its two branches of bifurcation, are incorporated into the atrial wall, and the four branches (two for each lung) come to open directly into the atrium.

4. Origin of the aorta and pulmonary artery. They arise early in embryonic life through division of the aortic bulb (bulbus, p. 3:33) . This is accomplished by two lateral ridges which meet and fuse in the midline.

a. Relative position. After they are formed, the two arteries are not parallel but arranged somewhat like an X; the more ventral of the two is the pulmonary artery, the other the aorta (crossing toward the right dorsal to the pulmonary).

b. Aortic and pulmonary valves. They arise from endocardial thickenings of the aortic bulb.

5. The ventricles. The ventricles are lateral outpocketings of the early single ventricle. Their separation is accomplished by the formation of the:

a. Interventricular septum, which arises at the time of division of the aortic bulb, as a median elevation extending to the ventral endocardial cushion.

b. Interventricular foramen. This is a temporary communication between the right and left ventricles.

c. Septum membranaceum. It closes the interventricular foramen and completes the partition of the single ventricle.

6. The atrio-ventricular valves. They arise from the endocardial cushions which by fusion convert the single atrio-ventricular canal into two canals. Endocardial folds at the margins of these canals form the flaps of the valves, which become attached to the muscular trabeculae of the inner ventricular wall.

D. Anomalies. They are rather frequent. Among the most important are:

1. Dextrocardia, or transposition of the heart, usually associated with general inversion of the viscera (situs inversus).

2. Incomplete ventricular septum due to deficiency of the septum membranaceum.

3. Persistence of the foramen ovale due to improper fusion of the septum primum and secundum. When the blood of the two sides mixes it causes cyanosis, seen in the “blue baby.”

IV. Development of the Arteries

The first arteries to appear in the embryo have already been mentioned, as well as the presence of aortic arches. The transformations of the latter are of great importance.

A. Transformation of the aortic arches.

1. Number of arches. In human embryos there are five pairs of aortic arches, which are numbered first, second, third, fourth and sixth since the fifth, present in other animals, never develops fully. They are not all present at any one time, due to early degeneration of the first and second.

2. Internal carotids. They are cephalic portions of the dorsal aortae after the disappearance of the first and second arches. They continue growing cephalad to enter the head.

3. External carotids. Each arises from the third arch, the proximal part of which becomes the common carotid. The distal part of the arch joining with the dorsal aorta becomes the proximal segment of the internal carotid.

4. Fourth arches. They also persist.

a. Left side. It becomes the arch of the aorta. Proximally, the short left ventral aorta is added to it.

b. Right side. The fourth arch arises from the enlarged right ventral aorta (now called the innominate) and constitutes the proximal part of the right subclavia. The middle part of the latter is the portion of the left aorta between the fourth arch and the vicinity of the point of fusion of the two aortae, while the distal part is a new growth arising from the caudal end of the middle portion at the level of the limb bud.

5. Left subclavia. This springs directly from the left dorsal aorta at the level of the corresponding limb bud but its position is shifted cephalad in later stages of development.

6. Sixth (pulmonary) arches. They arise from the pulmonary artery after its separation from the aorta (p. 135) and connect with the dorsal aortae.

a. Right side. A branch entering the corresponding lung bud arises about the middle of the arch. The portion of the latter between the origin of the branch and the right aorta degenerates.

b. Left side. A similar branch for the left lung bud is given off, but the portion of the arch between the branch and the left aorta remains as the:

c. Ductus arteriosus (Botalli) which becomes the arterial ligament of the adult.

B. Branches of the dorsal aorta. The aortae give off dorsal, lateral and ventral branches. The most important of each are:

1. Dorsal (intersegmental) arteries.

a. Vertebral artery. It arises from the subclavia. The two vertebrals join under the brain with the basilar artery.

b. Intercostal and lumbar arteries. They are the ventral rami of the dorsal intersegmental arteries.

2. Lateral arteries. They give rise to the renal, suprarenal, inferior phrenic and internal spermatic and ovarian arteries.

3. Ventral branches. The most important are the vitelline, coeliac, superior and inferior mesenteric, and the paired umbilicals.

V. Development of the Veins

The first paired veins to develop are the vitelline, umbilical and cardinals (p. 132). They undergo a series of transformations leading to the venous plan of the adult.

A. The vitelline (omphalomesenteric) veins. Their course is interrupted by growth of the liver which divides them into a large number of sinusoids. Each vein has a distal segment (from the yolk sac to the liver) and a proximal (from the liver to the corresponding horn of the sinus venosus).

1. The hepatic veins. They arise from the proximal parts of the vitelline veins.

2. Fate of the distal segments. They communicate with each other by three transverse anastomoses: a cranial (within the liver) and two dorsal and ventral to the duodenum, respectively. The more cranial portion of the left vitelline (within the liver) and the middle portion of the right drop out. What remains is shaped like an S.

a. Formation of the superior mesenteric. This is a new vessel which develops in the mesentery of the intestinal loop and joins the left vitelline vein near its middle anastomosis.

b. The portal vein. The persisting portion of the left vitelline vein and the portion of the right between the middle and cranial anastomosis become this vessel.

B. The umbilical veins. As the liver expands, its lateral surfaces engulf the umbilicals, which then send their blood to the heart by the more direct route of the liver sinusoids.

1. Fate of the right umbilical. When all the umbilical blood enters the liver the entire right umbilical vein atrophies.

2 . Left umbilical. Its proximal segment also atrophies.

3. Formation of the ductus venosus. This arises through enlargement of some of the hepatic sinusoids. It communicates with the left umbilical and opens into the common hepatic vein.

4. Fate of the ductus venosus. After birth it is obliterated and forms the solid venous ligament.

5 . Fate of the left umbilical vein. Through a similar obliteration its remnant, from the navel to the liver, constitutes the ligamentum teres.

C. The cardinal veins.

1. Anterior cardinals. An oblique anastomosis between these veins gives rise to the :

a. Left innominate vein, which increases in diameter as the proximal portion of the left cardinal atrophies.

b. Superior vena cava. The right common cardinal and anterior cardinal as far as the oblique anastomosis become the superior vena cava.

c. Right innominate vein. The portion of the right anterior cardinal between the anastomosis and the right subclavian.

d. Internal jugulars. They are the distal segments of the anterior cardinals.

e. External jugulars and subclavian veins. They develop independently and later open into the anterior cardinals.

2. Posterior cardinals, subcardinals and supracardinal veins. These three sets of veins appear successively in the order mentioned.

a. Postcardinals. They develop primarily as the veins of the mesonephroi and disappear as these organs wane.

b. Subcardinals. They anastomose in the midline; the anastomosis forms the left renal vein.

c. Supracardinals. They unite by a transverse anastomosis and become the azygos and hemiazygos veins.

3. The inferior vena cava. It consists of four segments arising from different sources:

a. * An hepatic segment, derived from the hepatic vein and sinusoids; it connects with the right subcardinal through a vein in the caval mesentery (p. 130).

b. A prerenal segment, formed from the right subcardinal.

c. A renal segment, comprising an anastomosis between the right subcardinal and right supracardinal veins.

d. A supracardinal segment, from the lumbar portion of the right supracardinal vein.

VI. Fetal Circulation and Changes at Birth

A. Course followed by the blood in the fetus. Contrary to formerly held views the oxygenated blood reaching the fetus through the umbilical vein becomes mixed with venous blood from diverse sources.

1. Source of the oxygenated blood. This is the placenta, where the venous blood conveyed to the chorionic villi by the umbilical arteries becomes arterial.

2 . Return of the oxygenated blood. By way of the left umbilical vein it enters the ductus venosus and reaches the right atrium through the inferior vena cava.

3 . Mixing of the blood. The venous blood of the portal vein and inferior vena cava contaminates the oxygenated blood; a further mixture of bloods takes place in the right atrium, which receives venous blood through the superior vena cava.

4. Passage through the heart. The mixed blood which has entered the right atrium follows two different courses:

a. Through the foramen ovale to left atrium, and through the aorta to the head and body.

b. Through the right atrio-ventricular foramen to the right ventricle and hence to the aorta through left pulmonary artery and ductus arteriosus.

B. Changes at birth. The placental circulation ceases when the lungs become functional. The chief events following this change are:

1. Gradual closure of the foramen ovale (p. 134) resulting from equalization of the pressures in the two atria.

2 . Obliteration of the ductus arteriosus (p. 137) following increased diversion of blood from the pulmonary trunk to the lungs.

3. Rapid obliteration of the umbilical vein, whose fate has been indicated (p. 138). The arteries become the lateral umbilical ligaments.

4. Atrophy of the ductus venosus, and its transformation into the ligamentum venosum.

The Urogenital System

The urinary and reproductive systems are closely associated in development. Both arise from mesoderm of the same region as a common urogenital fold which is soon divided into nephric and genital ridges.

I. The Urinary Organs

In the course of evolution the vertebrates have developed three types of kidneys: the pronephros, present in Amphioxus and certain lampreys; the mesonephros, functional throughout life in fishes and amphibians; and the metanephros or definitive kidney of reptiles, birds and mammals. The three types occur in a sequence during the development of the higher vertebrates.

A. Pronephros. In the human it consists of about seven pairs of rudimentary tubules.

1. Origin. They are formed as dorsal outpocketings of the intermediate cell mass (nephrotome) of the 7th to 14th somites. The first formed tubules degenerate before the last appear.

2. Pronephric ducts. The tubules of each side open into a longitudinal collecting tube which reaches the lateral wall of the cloaca, in which it opens.

B. Mesonephros (Wolffian body). This is larger than the pronephros and serves as a temporary excretory organ. It is constituted by many tubules (up to 80) which arise cranially as far as the 6th cervical segment.

1. Differentiation of the tubules. The free end of the early Sshaped tubule is dilated, and its walls become thin. The proximal end is united with the pronephric (now mesonephric) duct.

a. Formation of the glomeruli. A knot of looped blood vessels pressing on one of the hemispheres of the dilated portion causes its invagination into the other hemisphere.

b. Bowman's capsule. This is the double-walled capsule produced by the invagination mentioned above. The capsule and glomerulus together constitute a mesonephric (Malpighian) corpuscle.

c. Tubular portion. Each tubule shows a light staining secretory portion and a thinner, more deeply stained collecting part opening into the mesonephric duct.

d. Position. The glomeruli are mesially placed, the ducts occupy a lateral position while the tubules are largely dorsal.

C. Metanephros (permanent kidney). This arises in the pelvic region and it has a double origin. The ureter, pelvis and collecting tubules are outgrowths of the mesonephric duct; the secretory tubules and glomeruli develop from the caudal end of the nephrogenic cord.

1. The ureteric bud. This arises from the mesonephric duct; it grows at first dorsad, then turns cephalad. Its proximal, elongated portion is the ureter, the distal expanded portion the renal pelvis.

2. Formation of collecting tubules. These grow out from the primitive renal pelvis. Through branching they give rise to secondary, tertiary, quaternary, etc., tubules until about 12 generations have been produced; the tubules of the 5th order become papillary ducts (p. 72). The collecting tubules form a large part of the medulla.

3 . Differentiation of the nephrogenic blastema. This forms a cap about the primitive pelvis and is carried along with it during the elongation of the ureteric bud.

a. Formation of the lobes. The nephrogenic blastema covers the ends of the newly formed collecting tubules tributary to a primary tubule; in this way the cortex is subdivided into lobes by grooves. The external lobation gradually disappears after birth.

b. Formation of the secretory tubules. They arise from the blastema. The first few generations degenerate; new ones are produced near the surface of the organ.

4. Union of the secretory and collecting portions of the tubules. They unite secondarily into continuous tubules. Failure of this union leads to congenital cystic kidney.

D. Differentiation of the cloaca. In early human embryos the cloaca receives laterally the mesonephric ducts, dorsally the hind-gut, while its cephalic end gives off the allantois.

1. Division. It is accomplished through the development of the cloacal septum which, pushing caudad, separates the dorsal rectum from the ventral urogenital sinus.

2. Primitive perineum. This is the exposed tip of the septum, after rupture of the cloacal membrane (p. 121) which it reaches.

3 . Differentiation of the urogenital sinus. By elongation and constriction the sinus is divided into:

a. The vesico-urethral portion, which receives the mesonephric ducts and ureters and is continuous with the allantois.

b. The phallic portion, connected with the former by a narrow constriction and extending into the genital tubercle of both sexes (p. 147). It becomes the cavernous urethra of the male but it is merely merged with the vaginal vestibule of the female

(P- I 47) 4. Differentiation of the vesico-urethral portion. The enlarging bladder takes up into its walls the proximal ends of the mesonephric ducts to a level beyond the origin of the ureters; the four ducts thus acquire separate openings.

a. The ureters, open more laterally into the saccular bladder.

b. The mesonephric ducts are displaced caudad and come to open into the dorsal wall of the urethra on a hillock (Muller’s tubercle).

c. Urachus. This is the apex of the bladder continuous with the allantoic stalk at the umbilicus; after birth it constitutes the middle umbilical ligament.

II. The Genital Organs

The early development of the genital organs is identical in the two sexes. Each embryo develops a male and female system of ducts; after the sex is definitely established the ducts of the opposite sex degenerate.

A. The gonads. This term is applied to the primordial sex glands during their early indifferent stage.

1. Origin. They arise from the genital fold, which separates from the mesonephric fold in early phases of development.

2. Structure. The indifferent gonad consists of:

a. The germinal epithelium, of cuboidal cells forming one or more layers.

b. An inner epithelial mass of anastomosed strands derived from the germinal epithelium. The cords are separated by mesenchyme and contain scattered germ cells.

B. Differentiation of the testis. This happens after the 6th week.

1. The testis cords. These are branched and anastomosed strands proliferated from the germinal epithelium. They consist of indifferent cells with a few larger germ cells.

2. The albuginea. It arises from mesenchyme that penetrates between the germinal epithelium and the testis cords.

3. Fate of the germinal epithelium. It is changed into ordinary mesothelium.

4. The rete testis. The testis cords converge toward the mesorchium and connect with the dense primordium of the rete testis.

5. The seminiferous tubules. The cord cells gradually arrange themselves as a stratified epithelium lining lumina continuous with the lumina of the rete. Spermatogonia arise from indifferent cells. The proximal portions of the tubules remain straight (tubuli recti).

C. Differentiation of the ovary. The differentiation of the ovary takes place later than in the case of the testis.

1. Division into cortex and medulla. The inner epithelial mass of the indifferent gonad becomes less dense centrally to produce the medulla, while near the periphery it constitutes a denser cortex. Primordial germ cells occur in both zones, but they predominate in the medulla.

2 . The rete ovarii. This is the homologue of the rete testis and arises from a dense primordium.

3. The second proliferation. After the 3rd month the ovary grows rapidly, owing to the formation of a new cortex probably derived through proliferation of the germinal epithelium.

4. The albuginea. After the second proliferation the albuginea differentiates beneath the germinal epithelium, which does not become mesothelium but remains as a layer of cuboidal or low columnar cells (p. 81).

D. Transformation of the mesonephric tubules and ducts. The involution or degeneration of the mesonephros spares a number of mesonephric tubules, which remain connected with the sex glands in the two sexes. They form a cranial and a caudal group.

1. In the male.

a. Cranial group. Most of the cranially placed tubules (9 to 15) become connected with the tubules of the rete testis to form the ductuli efferentes, but a few of the most cranially placed form the appendix of the epididymis.

b. Caudal group. This, although composed of vestigial tubules, persists as the coiled, blindly ending tubules of the paradidymis, and the aberrant ductules.

c. Mesonephric duct. Its upper end coils into the duct of the epididymis, while the caudal portion remains straight and extends from the epididymis to the urethra as the ductus deferens and ejaculatory duct.

d. Ampulla. It develops near the opening of the ejaculatory duct into the urethra; the seminal vesicle is an outpocketing of the ampulla.

2 . In the female. Although the rete ovarii is vestigial it is retained in the adult.

a. Cranial group. Most tubules of this group form the epoophoron, but a few, cranially placed, become the cystic vesicular appendages associated with the fimbria; they are the homologues of the efferent ductules and appendix of the epididymis.

b. Caudal group. They constitute the more inconstant paroophoron, the homologue of the paradidymis and ductuli aberrantes of the male.

c. Mesonephric duct. Its greater part atrophies; the persisting portions are the ducts of the paroophoron (Gartner’s ducts) present in the region of the uterus and vagina; they correspond to the duct of the epididymis, ductus deferens, seminal vesicle and ejaculatory duct of the male.

E. The Mullerian ducts.

1. Origin. They first appear as a ventro-lateral groove in the thickened epithelium of each urogenital fold, near the cephalic pole of the mesonephros.

2. Closure of the groove. The cranial end of the groove remains open, while the rest closes into a tube which separates from the epithelium, beneath which it comes to lie.

3. Opening into the cloaca. The solid end of the tube grows caudad, beneath the epithelium and lateral to the mesonephric duct. The tubes meet in the midline and penetrate the dorsal wall of the uro-genital sinus, along with the mesonephric ducts.

4. Fate in the two sexes.

a. In the female.

(1) Uterine (Fallopian) tubes. They arise from the cranial portions of the ducts.

(2) Uterus. This originates from the next portion of the ducts, which fuse into a single tube. The thick muscular walls of the uterus are foreshadowed by the presence of a thick layer of mesenchyme around the epithelial portions of the tubes.

(3) Vagina. The upper two thirds are probably formed through fusion of the Mullerian ducts in the midline. The lower third arises from the uro-genital sinus.

b. In the male. Degeneration of the Mullerian ducts begins with the third month and only the extreme cranial and caudal ends are spared.

(1) Cranial end. It becomes the appendage of the testis.

( 2 ) Caudal end. It persists as a small pouch on the dorsal wall of the urethra, the utriculus prostaticus or masculine vagina.

F. Descent of the testis and ovary. Their original positions gradually change during development. At first they extend caudad from the diaphragm, but later they are shifted to a more caudal position.

1. Testes. Their caudal ends come to lie at the boundary between abdomen and pelvis. This early migration is followed by their descent into the scrotal sacs.

a. Formation of the vaginal processes. These arise early in the third month. Each is an outpocketing of the abdominal cavity which passes over the pubis, then through the inguinal canal into the corresponding scrotal sac.

b. The gubernaculum testis. A continuous ligament extending from the caudal end of the testis through the inguinal canal to the scrotal integument.

c. Penetration into the scrotal sacs. During the 8th month shortening of the gubernacula draws the testes into the scrotum. Each testis is still retro-peritoneal (i.e. it is covered by the wall of the processus vaginalis) so it lies outside the cavity of the latter. Failure of the testis to enter the scrotal sac causes cryptorchism.

d. Obliteration of the canal of the vaginal process. After birth this narrow canal, connecting the vaginal process with the abdominal cavity, disappears.

e. Tunica vaginalis. The now isolated vaginal process or sac represents the tunica vaginalis of the testis; its visceral layer closely invests the testis whereas the parietal lines the scrotal sac.

f. Spermatic cord. The ductus deferens and spermatic vessels and nerves are carried down into the scrotum along with the testis and epididymis. They are surrounded by connective tissue and constitute the spermatic cord.

2. Ovaries. After their early migration they come to lie within the pelvis, where each rotates until it is placed in a transverse position.

G. The external genitalia.

1. Indifferent stage. Up to the beginning of the 8th week the external genitalia are identical in the two sexes.

a. Genital tubercle. This round eminence develops in the ventral body wall between the umbilical cord and the tail.

b. Urethral groove and folds. It is located on the caudal surface of the tubercle, and is separated from the anus by the primitive perineum. The margins of the groove are the urethral folds.

c. Phallus. This is visible by the end df the 7th week as a cylindrical prolongation ending distally as a rounded glans.

d. Labio-scrotal swellings. They occur on each side of the base of the phallus, from which they are separated by a groove.

2 . Transformation in the two sexes. The fate of the parts just mentioned differs according to the sex, which cannot be recognized for sure until the end of the 10th week.

a. Male.

(1) Formation of the urethra. This is accomplished through transformation of the urethral groove into a hollow tube; the fused edges of the groove constitute the raphe.

(2) Migration of the scrotal swellings. These shift caudad and each becomes a half of the scrotum, separated from the other half by the raphe and the underlying scrotal septum.

(3) Elongation of the penis. It is accompanied by a continuation of the formation of the urethra, which finally reaches the glans.

(4) Corpora cavernosa. They arise as columns of mesenchyme within the shaft of the penis.

b. Female. The changes are less marked and take place much more gradually.

(1) Phallus. It lags in development and becomes the clitoris; its distal portion is the glans clitoridis.

(2) Urethral groove. This never reaches the glans, as in the male, but remains open as the vestibule.

(3) Urethral folds. They become the labia minora.

(4) Labio-scrotal swellings. They grow caudad and fuse in front of the anus as the posterior commisure, while their lateral portions are converted into the labia majora.

H. Anomalies. True hermaphroditism in man is very rare; false hermaphroditism, characterized by the presence of the genital glands of one sex with external genitalia and secondary sexual characteristics of the other, is much more frequent. When the lips of the uro-genital sinus in males fail to fuse hypospadias result, a common feature in hermaphroditism of the female type.

The Skeletal System

The supporting tissues (connective tissue, cartilage and bone) arise from mesenchyme, which consists of irregularly branched cells separated by uneven spaces filled with a fluid resembling lymph.

I. Connective Tissue

The mesenchyme cells become fibroblasts.

A. Origin of the fibers. The characteristic connective tissue fibers arise in the intercellular spaces rather than within the fibroblasts, as was formerly supposed.

1. Argyrophil fibers. These are the first to appear, remaining as such in the reticulum of certain organs (spleen, liver, lymph nodes).

2. Collagenous fibers. They arise through chemical transformation of argyrophil fibers which are aggregated into bundles.

3. Elastic fibers. Their development is not exactly known; they are laid down amongst the collagenous fibers.

B. Adipose tissue. Certain mesenchyme cells, called lipoblasts, give rise to fat cells. Fat droplets appear in their cytoplasm and coalesce into a large drop which pushes the nucleus to the periphery of the cell.

II. Cartilage

The mesenchymal cells which will give rise to cartilage lose their processes and are aggregated into a mass of polygonal cells, known as precartilage. The intercellular substance appearing between the cells becomes the ground substance or matrix of the cartilage and the cells are enclosed within lacunae.

III. Bone. Development of the Skeleton

The histogenesis of bone has already been described in the Histology (p. 18 ). From the standpoint of Embryology the skeleton is composed of two portions, the axial and appendicular skeleton, respectively.

A. Axial skeleton. This comprises the vertebral column and ribs, the sternum and the skull.

1. Vertebrae. The vertebral column and ribs originate from the sclerotomes of the somites (p. 109), which consist of spindleshaped mesenchymal cells.

a. Blastemic stage. Each sclerotome differentiates into a caudal dense half, and a cranial less dense portion. The dense portion of each sclerotome mass later joins the looser cranial mass of the sclerotome right caudal to it, to form the substance of the vertebra.

(1) Formation of the body of the vertebra. The two sclerotomic portions enclose the notochord to form this part.

(2) Vertebral arch. From the dense half, dorsal extensions grow around the neural tube.

(3) Costal processes. These are ventro-lateral outgrowths.

(4) Intervertebral disks. They arise from mesenchyme derived from the dense portion of the sclerotome.

b. Chondrification. There are six centers: two in the vertebral body, one in each half of the vertebral arch, and one in each costal process. They enlarge and fuse into a solid cartilaginous vertebra.

c. Ossification. This occurs during the 10th week. There is a single center for the body of the vertebra and one in each half of its arch; the union of these parts is not completed until several years after birth.

2. Ribs. They arise from the costal processes; their original union with the vertebra is replaced by a joint which receives the head of the rib. The transverse process of the vertebra extends outward and articulates with the growing tubercle of the rib. There is a single center of ossification for each rib.

3. Sternum. Arises from paired sternal bars which unite the upper eight or nine cartilaginous thoracic ribs of each side. At an early period the two bars fuse together; this is followed by ossification.

4. The skull. Most of the bones of the skull arise from the chondrocranium; the flat bones of the vault and face (frontal, parietals, nasals, lacrimals, zygomatics and vomer) are purely membranous, whereas the occipital, sphenoid and temporals are mixed.

a. The chondrocranium. The chondrocranium is a continuous mass of cartilage extending from the occipital to the ethmoidal region and to a certain extent dorsally at the sides and behind. (1) The periotic (auditory) capsules. They are formed by cartilage which encloses the internal ear. They fuse with the chondrocranium.

(2) Ossification. Most cartilage bones of the skull develop from two or more formative centers. The ossification begins early in the third month. The occipital has four centers of ossification, the sphenoid five, the ethmoid four, and the temporal probably two.

b. Membrane bones of the skull. The frontal develops from two centers on each side of the midline, the parietals from one center each, the vomer from two, the nasal, lacrimal and zygomatic from one center each.

c. Branchial arch skeleton. This is formed by cartilage and membrane bones derived from the branchial arches.

(1) First (mandibular) arch. A cartilaginous bar (Meckel’s cartilage) develops only in the mandibular process; the maxillary process has no cartilaginous skeleton but two membrane bones, the palatine and maxilla are developed in it, the former from one center, the maxilla from two or possibly more.

(a) Proximal portion of Meckel’s cartilage. It extends into the tympanic cavity, where it forms two of the bones of the middle ‘ear, the malleus (hammer) and incus (anvil).

(b) Distal portion. This is invested by membrane bone which forms the body and rami of the jaw; the membrane bones are paired, and fuse in the midline in a symphysis. The invested portion of Meckel’s cartilage degenerates.

(2) Second (hyoid) arch. Its cartilage also enters the periotic capsule; this proximal segment gives rise to the stapes (stirrup) of the middle ear, the styloid process of the temporal, and the lesser horn of the hyoid bone. Between the latter and the styloid process failure of ossification produces the stylohyoid ligament.

(3) Third branchial arch. It produces the greater horns of the hyoid, while the plate (copula) connecting the two arches becomes the body of this bone.

(4) Fourth and fifth branchial arches. They differentiate into the cartilages of the larynx.

B. Appendicular skeleton. It is derived from the somatic mesenchyme which forms the core of the limb bud and becomes converted into cartilage; ossification of the latter produces all the bones of the limbs, with the possible exception of the clavicle.

1. Pectoral girdle and arm.

a. The clavicle is the first bone of the skeleton to ossify; it has two centers of ossification.

b. The scapula has two chief centers, one for the body and spine, the other for the coracoid process, and several later epiphyseal centers.

c. The humerus, radius and ulna all ossify from a single primary center in the diaphysis and an epiphyseal center at each end.

d. Carpals, metacarpals and phalanges. Each carpal ossifies from a single center; the metacarpals and phalanges also have a single primary center and an additional epiphyseal center.

2. Pelvic girdle and leg.

a. The innominate arises from three main centers of ossification, one for the ileum, one for the ischium, and another for the pubis. The three join in the acetabulum, which receives the head of the femur.

b. Femur, tibia and fibula. Their development is similar to that of the corresponding bones of the arm. The patella is regarded as a sesamoid bone.

c. Tarsal, metatarsals and phalanges. They develop as the corresponding bones of the hand.

The Muscular System

The histogenesis of the three varieties of muscle has been already dealt with (pp. 21, 22, 24).

I. The Visceral Musculature

The muscle (smooth) associated with the hollow viscera arises from the splanchnic mesoderm (p. 109). The same layer originates the cardiac (striated) variety, which develops from the thickened epimyocardial lining on the outer surface of the cardiac tubes (p. 133).

The Skeletal Musculature

Most of the skeletal muscles originate from the myotomes. The differentiation of the skeletal muscles takes place rapidly, and at about the 8th week they are already capable of correlated movements.

A. Changes in the myotomes.

1. Migration, wholly or in part, to more or less distant regions.

2. Fusion of portions of successive myotomes into a single muscle. With the loss of the segmental arrangement the original innervation of each portion of a myotome is retained throughout life.

3. Splitting. This may be longitudinal or tangential. In the first case the myotome gives rise to several subdivisions, in the second there is an increase in the number of layers. With the splitting there may be a change in the direction of the fibers.

4. Degeneration of myotomes or parts of myotomes; the degenerated portions may be changed into ligaments, fascias and aponeurosis.

B. Muscles of the trunk.

1. The lateral and abdominal muscles originate as ventral extensions of the myotomes; the somatic mesoderm gives rise only to the intermuscular connective tissue.

2. The superficial portions of a dorsal longitudinal column of fused myotomes on each side produce the various long muscles of the back, innervated by the dorsal rami of the spinal nerves.

3. The intervertebral muscles develop from the deeper, non-fused portions of the myotomes.

C. Muscles of the neck. The long muscles arise from the same longitudinal column producing the corresponding muscles of the trunk, and are also innervated by dorsal rami. Other muscles differentiate from ventral extensions of the cervical myotomes and from the branchial arches.

D. Muscles of the head. The head lacks definite somites but it is possible that the mesenchyme which gives rise to the eye muscles — supplied by somatic motor nerves (III, IV and VI) — is of myotomic origin. The other muscles of the head develop from the lateral mesoderm and retain their primitive branchial arch innervation.

1. First (mandibular) arch. Gives rise to the muscles of mastication (temporal, masseter, anterior belly of digastric, mylohyoid and pterygoids) and other muscles associated with the trigeminal (fifth) nerve (tensors of palatine velum and tympanum).

2. Second (hyoid) arch. Produces the muscles of expression (facial muscles) and all other muscles supplied by the facial (seventh) nerve (stylo-hyoid, posterior belly of digastric, stapedius, platysma, occipito-frontal).

3. Third arch. This is the source of muscles supplied by the glossopharyngeal (ninth) nerve (stylo-pharyngeus and part of the pharyngeal constrictors).

4. Fourth and fifth arches. They give rise to part of the pharyngeal constrictors, certain muscles of the palate and the muscles of the larynx, all of which receive their supply from the vagus (tenth) nerve. The accessory (eleventh) nerve innervates the sterno-mastoid and trapezius, regarded as branchiomeric muscles.

E. Muscles of the limbs. The direct myotomic origin of these muscles in mammals is questionable even though there are indications of migration of mesenchyme cells from the edges of the cervical myotomes. The development of the upper limb muscles is more advanced than those of the lower limb, and the proximal muscles appear earlier than the distal in any case.

Ectodermic Derivatives

The Integument And Its Derivatives

Although it is included among the ectodermic derivatives the integument has a double origin.

I. The Skin

The superficial epithelium arises from ectoderm; the derma or corium from mesoderm.

A. Epidermis. In early embryos it is a single layer of cuboidal cells, but it soon becomes double-layered.

1. Epitrichium (periderm). This is the most superficial layer, composed of flattened cells. The term ‘epitrichium’ alludes to the fact that the layer is lifted off by the growing hairs. Failure of desquamation leads to ichthyosis, a condition which is frequently hereditary.

2. Basal layer. Made up of cuboidal cells which will give rise to the other layers of the epidermis. Stratification occurs after the 4th month.

3. Vernix caseosa. This is a mixture of desquamated epidermal cells, lanugo hairs and sebaceous secretion which covers the fetal skin and prevents its maceration by the amniotic fluid.

B. Derma. In most vertebrates it arises from the dermatomes of the somites (p. 109), but it has been claimed that dermatomes are absent in mammalian embryos. The derma, therefore, probably arises from mesenchyme.

II. The Hair

The earliest hairs begin to appear at the end of the second month on the eyebrows, upper lip and chin; later (4th month) the body hair develops. The latter is at first fine and silky, and is known as lanugo.

A. Development. A hair follicle begins as a cluster of basal layer cells in the epidermis.

1. Elongation. The cluster becomes elongated and gradually sinks into the underlying derma.

2. Differentiation. The base of the hair primordium enlarges into the bulb, which fits as a cap on the surface of the mesenchyme mass which will become the papilla.

3. Structure. The follicle at this stage consists of two layers :

a. An outer layer of columnar cells continuous with the basal layer of the epidermis; they give rise to the outer sheath.

b. A core of polyhedral cells, which produces the substance of the hair.

c. Connective tissue sheath. This arises from mesenchyme.

4. Growth of the hair. The hair is a proliferation from the basal epidermal cells next to the papilla. They produce an axial core which becomes the inner sheath and shaft, respectively.

III. The Sebaceous Glands

Most sebaceous glands develop in connection with the hair follicles. They arise as solid epidermal buds which become lobulated.

IV. The Sweat Glands

Some develop in connection with hair follicles, from which they separate later, but most appear independently as solid downgrowths from the epidermis. The simple, cylindrical downgrowths coil and acquire lumina. The walls of the tubules consist of:

A. An inner layer of cuboidal cells, which become the glandular elements.

B. An outer layer, whose cells become transformed into myo-epithelial, contractile elements (p. 48) .

V. The Mammary Glands

They are usually regarded as modified sweat glands and appear early in the two sexes as longitudinal ectodermal thickenings which extend on each side between the bases of the limb buds. Each ridge is called the milk line.

A. Disappearance of the milk line. In man the milk line is best seen in the pectoral region; the more caudal portion soon disappears. If persistent it gives rise to accessory mammary glands.

B. Development of the glands. Each gland begins as a downgrowth from the milk line in the region of the future breast. The primordium gradually grows into 15-20 cords (primary milk ducts) which branch in the derma and give rise to acini.

C. Development of the nipple. Where the milk ducts open the epidermis is raised to form the nipple; this may not happen until after birth.

The Nervous System

The central nervous system develops from the neural plate, an ectodermic band along the mid-dorsal line of the embryo (p. 108).

I. The Central Nervous System

A. Histogenesis. The neural tube gives rise to all the nervous tissues except the cerebrospinal and sympathetic ganglia, and the olfactory neurones (p. 95). The primitive cells of the tube differentiate into two kinds of elements, namely, the nerve cells and the supporting cells.

1. Early differentiation of the neural tube. Its wall shows at first several indistinct layers; later it becomes separated into three distinct zones:

a. Inner (ependymal) zone, near the lumen of the tube from which it is separated by a thin internal limiting membrane; its cells send processes toward the periphery. It constitutes most of the roof and floor of the neural canal (roof and floor plates).

b. Middle (mantle) zone, consisting of many, closely packed cells; it becomes the gray matter, and contributes to the marked thickening of the lateral walls of the neural tube.

c. Marginal zone, largely non-nucleated and gradually invaded by the axons of the cells of the preceding zone; it becomes the white matter. Externally it is bounded by the outer limiting membrane.

d. Sulcus limitans. A groove on the inner surface of each lateral wall subdivides the latter into a dorsal alar plate (sensory) and a ventral basal plate (motor).

2 . Differentiation of the neuroblasts. The neuroblasts are the embryonic nerve cells which become the neurones of the adult. They arise from germinal cells which occur in the ependymal zone and divide by mitosis.

a. Development of efferent neurones. The neuroblasts become pear-shaped and from the narrow end of the cell a slender axon grows. Later they acquire neurofibrils and develop dendritic processes. Many axons leave the spinal cord as ventral roots.

b. Development of the ganglia. They arise from the :

(1) Neural (ganglion) crest, which is a longitudinal ridge of cells on each side, where the ectoderm joins the wall of the neural groove.

(2) Migration of the crest. After closure of the neural tube (p. 108) the neural crests separate into right and left halves which occupy a position between the tube and the dorsal portions of the myotomes.

(3) Segmentation into spinal ganglia. The continuous neural crest bands caudal to the otocysts soon show swellings which become spinal ganglia. The portions of the crest between the ganglionic swellings disappear.

(4) Formation of the cranial ganglia. These also develop from the crest but are not segmentally arranged. Certain cranial ganglia receive contributions from ectodermal thickenings called placodes.

(5) Formation of sympathetic ganglia. They are believed by some to originate from the neural crest.

c. Development of afferent neurones. The cells of the cerebrospinal ganglia differentiate into ganglion cells (afferent neurones) and supporting cells.

(1) Bipolar stage. The ganglionic neuroblasts become spindle-shaped, and are transformed into bipolar neurones (p. 24) through growth of a process at each end. They remain in this condition in the auditory ganglia (p. 31).

(2) Dorsal roots. They are formed by processes (axons) directed toward the neural tube. They enter the latter and bifurcate into an ascending and a descending process coursing in the dorsal region of the tube. Collaterals from the processes establish connection with the neurones of the mantle layer.

(3) Peripheral processes. These pass outward and join the axons of the efferent neurones of the cord coursing into the ventral roots; the common bundles thus constituted are the trunks of the spinal nerves.

(4) Transformation into monopolar neurones. While some neuroblasts remain in the bipolar stage most others are changed into monopolar neurones chiefly by fusion, for a variable distance from the cell body, of the two primary processes into a common stem.

3. Differentiation of the supporting elements. These arise from spongioblasts, which originate from the undifferentiated cells of the neural plate tissue.

a. In the neural tube. For some time the spongioblasts are elongated and radially arranged within the tube, with their nuclei placed close to the lumen; the inner end of the cell touches the internal limiting membrane, while the outer reaches the periphery of the tube.

(1) Ependymal cells. These are spongioblasts which retain their primitive shape and position.

(2) Neuroglia cells. Many of the elongated spongioblasts lose their connections with the lumen of the neural tube, and some of them also lose their peripheral portion, to become neuroglia cells (astroglia and oligodendroglia, p. 33).

b. In the ganglia. The supporting cells become capsule cells, satellite cells (p. 31), and sheath cells; the latter migrate peripherally along with the growing axons and envelop them as neurilemma or cells of Schwann (p. 27).

B. Morphogenesis. The formation of the neural tube and the subdivision of its anterior, expanded end into the three primary brain vesicles have already been described (p. 108). The rest of the tube is the spinal cord.

1. The spinal cord. The typical three zones described previously are clearly seen by the 5th week.

a. Shape. At first cylindrical it becomes enlarged at the level of the two nerve plexuses that supply the limbs (cervical and lumbar enlargements, respectively).

b. Decrease in growth. After the 3rd month the vertebral column grows faster than the spinal cord. Since the latter is anchored to the brain the caudal displacement of the vertebrae causes an elongation of the roots of the lumbo-sacral nerves, which together constitute the cauda equina.

c. Filum terminale. Since the posterior end of the neural tube retains its terminal connections during the period of unequal growth the caudal portion of the tube becomes this slender, fibrous strand which occupies the axis of the cauda equina.

d. Formation of the central canal. The neural canal is at first quite large and roughly diamond-shaped in cross section. Later the lateral walls fuse dorsally (i.e. above the sulcus limitans); in this way the dorsal portion of the canal is obliterated and the persisting ventral portion becomes the definitive central canal.

e. Differentiation of the walls. The thickening of the lateral walls of the early spinal cord dominates the final arrangement of the gray and white matter and is largely responsible for the disappearance of the roof plate and reduction in size of the floor plate.

(1) Formation of the dorsal median septum. This arises largely from the fused ependymal layers during obliteration of the dorsal portion of the neural canal.

(2) Formation of the ventral median fissure. The floor plate lags in development and since it is interposed between the rapidly thickening ventral portions of the lateral walls these fail to meet, giving rise to the fissure.

f. Anomalies. The spinal cord may be absent, or the neural tube may have failed to close; this condition often accompanies cleft spine which is really a defect of the vertebral column. A sac may protrude through the cleft and may be formed by the cord only (myelocele) or by the cord and its meninges (meningomyelocele).

2 . The brain. Of the three primary vesicles (p. 108) the first and last are subdivided into two secondary vesicles each: the telencephalon and diencephalon, in the case of the fore-brain (prosencephalon), and the metencephalon and myelencephalon, in the case of the hind-brain (rhombencephalon).

a. Cavities. With the subdivision of the primary vesicles the number of cavities is increased to four.

(1) Lateral ventricles. The cavity of the telencephalon extends into the paired hemispheres as the lateral (first and second) ventricles.

(2) Third ventricle. This is the cavity of the median portion of the telencephalon plus the cavity of the diencephalon.

(3) Fourth ventricle. It includes the merged lumina of the metencephalon and myelencephalon; it is continuous caudally with the central canal of the spinal cord.

(4) Cerebral aqueduct (of Sylvius). The mid-brain (mesencephalon) remains undivided; its primitive cavity becomes this narrow canal connecting the third and fourth ventricles.

b. Myelencephalon (medulla oblongata). It is the transition between the spinal cord and the brain. Its walls undergo certain differentiations.

(1) Roof -plate. This, instead of disappearing, as in the spinal cord, becomes the thin ependymal roof of the 4th ventricle.

(2) Chorioid plexus. Blood vessels grow into the layer of mesenchyme (tela chorioidea) which covers the outer surface of the ependymal roof and upon invagination of the latter they form this plexus.

(3) Lateral walls. The sulcus limitans persists, separating each wall into an alar and a basal plate.

(4) Floor plate. It persists and its ependymal cells elongate as the ventral wall of the myelencephalon thickens; the processes of the ependymal cells extend from the floor of the fourth ventricle to the ventral surface as the raphe.

(5) Nuclei of the alar plate. Neuroblasts arrange themselves into the terminal (receptive) nuclei of nerves V, VII, VIII, IX and X.

(6) Nuclei of the basal plate. Efferent neuroblasts form the motor nuclei of origin of nerves V, VI, VII, IX, X, XI, and XII.

c. Metencephalon. In general structure is similar to the preceding but it is the site of a marked embryonic flexure, and it also develops two specialized parts, the pons and cerebellum, respectively.

(1) Pontine flexure. This, although temporary, is highly characteristic of the embryo. Its convexity is ventrally directed. It disappears completely during fetal life.

(2) Roof-plate. Part of it is transformed into a thin plate of white matter both in front and behind the cerebellum, known as the anterior and posterior medullary velum, respectively; the other part merges with the cerebellum.

(3) Lateral walls. The still present sulcus limitans divides them into alar and basal plates. In the latter develop the motor nuclei of nerves V, VI, and VII, along with the reticular formation, present also in the myelencephalon. The alar plates contribute to the formation of the:

(4) Cerebellum. The plates assume a transverse position as the pontine flexure develops. Paired swellings near the midline foreshadow the vermis, while the lateral portions become the cerebellar hemispheres. The latter connect with the pons by means of the brachium pontis (middle cerebellar peduncle) .

(5) Pons. The pons develops as a thickening of the anterior wall of the pontine flexure.

(6) Floor plate. This forms the portion of the raphe within this region, where it is said to end.

d. Mesencephalon. This is the least-modified portion of the primitive brain tube.

(1) Roof plate. It becomes very narrow and finally disappears.

(2) Alar plates. They develop into the lamina that bears the corpora quadrigemina (superior and inferior colliculi), composed of stratified layers of neuroblasts.

(3) Basal plates. Their efferent neuroblasts form the motor nuclei of nerves III and IV. The tegmentum is to be regarded as an anterior extension of the reticular formation of the mete- and myelencephalon.

(4) Floor plate. It is absent in the mesencephalon and anterior to it.

(5) Cerebral peduncles. They occur on each side of the midline in the floor of the mesencephalon, and are composed of nerve fibers from the fore-brain and of sensory tracts coursing in the opposite direction.

e. Diencephalon. It lacks floor and basal plates, and nerves do not arise from it. It is connected ventrally with the hypophysis (pituitary body). Its cavity is the third ventricle.

(1) Roof plate. It becomes a thin plate in which the chorioid plexus of the third ventricle develops.

(2) Alar plates. Each of these is subdivided into three main regions :

(a) Epithalamus, at the junction of the caudal portion of the roof plate with the alar plate. The pineal gland (epiphysis cerebri) arises from this region.

(b) Thalamus. This is a marked swelling on the lateral wall; the two thalami may join each other in the midline through the massa intermedia. Ventrally is the:

(c) Hypothalamus, containing the infundibulum, tuber cinereum, and mammillary bodies.

(3) The hypophysis. This important endocrine has a double origin.

(a) Anterior lobe. It develops from an invagination of the roof of the stomodaeum just in front of the pharyngeal membrane (Rathke’s pouch). The invaginated ectodermic sac sinks beneath the epithelium, and becomes a hollow vesicle whose cavity is the residual lumen of the adult gland (p. 90).

(b) Posterior lobe (pars nervosa). This is the enlarged tip of the infundibular process, which grows as an invagination of the floor of the diencephalon and meets the hollow vesicle arising from Rathke’s pouch.

(4) Optic stalks. They are connected with each side of the diencephalon (p. 166).

f. Telencephalon. It becomes the most specialized and complex region of the mammalian brain. It consists of a median portion, continuous posteriorly with the diencephalon and containing the cephalic part of the third ventricle, and two lateral outpocketings, the cerebral hemispheres.

(1) Roof plate. It gives rise to the chorioid plexus.

(2) Alar plates. They produce practically the whole cerebral hemispheres.

(3) Cerebral hemispheres. They arise between the 5th and 6th week, and grow beyond the original rostral end of the neural tube, the wall of which is the lamina terminalis.

(a) Lateral ventricles. These are the cavities of the hemispheres; they communicate with the third ventricle through the paired interventricular foramina (of Monro).

(b) Corpus striatum. This is a thickening of the floor of the hemisphere. The groove separating it from the thalamus disappears and the two portions merge into a continuous mass.

(c) Internal capsule. Nerve fibers coursing between the striate body and the thalamus are gathered into a V-shaped lamina, open laterally. This is the internal capsule, which partly subdivides the corpus striatum into secondary masses (caudate and lenticular nuclei).

(d) Rhinencephalon. It is represented by the olfactory lobes which arise as swellings of the ventral surfaces of the hemispheres. The rostral or anterior part of each develops into the olfactory bulb and tract. Connected with the olfactory apparatus there is also a portion of the brain cortex (hippocampal system).

II. The Peripheral Nervous System

The formation and early differentiations of the neural crests have already been considered (pp. 108, 156).

A. The spinal nerves. Each nerve is attached to the cord by two roots: dorsal and ventral.

1. Dorsal root. It has a spinal ganglion associated with it. The neuroblasts send their axons into the marginal zone of the cord as dorsal root fibers; their peripheral processes join the ventral root fibers.

2 . Ventral root. This carries efferent fibers (axons of cells within the cord).

3. Nerve trunks. The mixed nerve trunks give off:

a. A dorsal ramus, supplying the dorsal skin and musculature; it continues as:

b. The ventral ramus which in turn sends a:

c. Ramus communicans, to the sympathetic; the ramus communicans carries efferent fibers (preganglionics).

d. The lateral and ventral terminal rami arise through division of the ventral ramus.

4. Plexuses. They are produced through anastomoses between the spinal nerves. The brachial and lumbo-sacral plexuses arise in this manner.

R. The cranial nerves. The cranial nerves are not segmentally arranged. There are twelve pairs of which three are purely (special) sensory, four are purely (somatic) motor while the other five are mixed (except the spinal accessory, purely motor in the adult but intimately associated with the vagus).

1. Special sensory:

a. Olfactory (I). It has no ganglion. For its termination see p. 96.

b. Optic (II). Consists of axons of neurones in the retina (p.

99) c. Auditory (VIII). Axons growing from the auditory ganglia (vestibular and cochlear).

2. Somatic motor:

a. Oculomotor (III). Nucleus of origin in the basal plate of the mesencephalon.

b. Trochlear (IV). Nucleus of origin as in the preceding, but more caudally placed.

c. Abducens (VI). Nucleus of origin in the pontine region of the metencephalon.

d. Hypoglossal (XII). Nucleus of origin in the basal plate of the myelencephalon; associated in embryonic life with rudimentary dorsal ganglia (of Froriep) which later disappear.

3. Visceral sensory and motor:

a. Trigeminal (V). Chiefly sensory; its main ganglion (semilunar or Gasserian ganglion) gives off three branches: ophthalmic, maxillary and mandibular. The motor nucleus sends fibers to the muscles of mastication.

b. Facial (VII). Chiefly motor; its sensory fibers are prolongations of neurones in the geniculate ganglion and end in the sensory organs of the tongue.

c. Glossopharyngeal (IX). Chiefly sensory; its motor fibers arise from the nucleus ambiguus, which it shares with the vagus. They innervate some of the pharyngeal muscles (p. 153). The sensory fibers are the peripheral processes of the superior and petrosal ganglia.

d. Vagus (X) and spinal accessory (XI). They occur as a complex.

(1) The motor fibers arise from nuclei in the spinal cord (spinal portion of accessory) and myelencephalon (bulbar portion of accessory; motor portion of the vagus). The accessory fibers soon separate from the vagus, which supplies motor fibers for the pharynx and larynx.

(2) The sensory fibers are processes of neurones residing in the jugular and nodose ganglia, respectively.

C. The sympathetic nervous system. Its origin is still a matter of discussion. The ganglia of the trunk develop before those of the head and neck region.

1. Formation of the sympathetic chains. The sympathetic primordia are first continuous neuroblastic bands; later the neuroblasts concentrate into segmentally arranged ganglia, connected by a longitudinal nerve cord (sympathetic trunk).

2. The collateral ganglia. The collateral ganglia (ganglia of the pre vertebral plexuses) develop later.

3 . Cranial sympathetic ganglia. The ciliary, spheno-palatine, and otic ganglia (parasympathetic) are not segmental and are derived mainly from the primitive semilunar (Gasserian) ganglion.

4. Chromaffin bodies. These, arising from cells in the primitive sympathetic ganglia which give the chromaffin reaction (p. 94), occur in close proximity to the ganglia (paraganglia) and in the abdominal sympathetic plexus. They gradually degenerate after birth, except the largest which is the:

5 . Suprarenal gland. It has a double origin:

a. Medulla. Its chromaffin cells (ectodermic) are derived from the coeliac plexus. They arise as masses of cells which invade the median side of the primordium of the:

b. Cortex. The cortex is of mesodermic origin and arises as proliferations of the peritoneal lining, on each side of the root of the mesentery. The early suprarenals are quite large and project from the dorsal coelomic wall, between the mesonephros and mesentery.

6. Carotid body (glomus caroticum). Although usually included among the paraganglia, its paraganglionic nature is doubtful since it has been shown to be a chemoreceptor (p. 30).

The Sense Organs

Only the specialized sense organs will be considered here.

I. The Organ of Taste

The taste buds arise as local thickenings of the tongue epithelium as well as the epithelium of the oral mucosa, pharynx and epiglottis. The cells of the thickening become the characteristic elements of the bud (p. 95). In late fetal life many of the taste buds degenerate and the adult distribution is attained.

II. The Nose

The early development of the nose has been considered in the section dealing with the formation of the face (p. 112).

A. Formation of the primitive choanae. The epithelial plates which separate the nasal fossae from the mouth cavity rupture caudally to produce these internal nasal openings. The nasal fossae now have outer (nostrils) and inner openings (choanae).

B. Formation of the lip and premaxillary palate. The front part of each epithelial plate is invaded by mesoderm, and becomes these parts.

C. The nasal septum. This arises from the medial fronto-nasal process (p. 1 12), which becomes narrower between the nasal fossae.

D. Separation of the nasal passages from the mouth cavity. It takes place after fusion of the palatine processes in the midline (p. 123). Fusion of the ventral border of the septum with the palate completes the separation of the nasal passages.

E. The permanent choanae. Their formation is finally accomplished by the fusion mentioned above. The permanent nasal passages consist, therefore, of the nasal fossae plus a portion of the primitive mouth cavity.

F. Vomero-nasal organs (of Jacobson). They are rudimentary epithelial sacs which open toward the front of the nasal septum. They usually degenerate in late fetal stages.

G. The conchae. They arise as folds on the lateral and medial walls of the nasal fossae; first cartilage, then bone develops in them.

H. The sinuses. They arise through absorption of bone. The spaces thus formed are soon lined by epithelium which evaginates from the nasal passages.

III. The Eye

Its development is complex. The sensory portion (retina) arises from the brain as optic vesicles, while the lens is an invagination of the ectoderm in front of each vesicle.

A. The optic vesicles. These are outpocketings of the fore-brain to which they are attached by narrower optic stalks.

1. The optic cups. They arise through invagination of the lateral hemisphere of the vesicle into the mesial hemisphere. A doublewalled cup is thus produced, connected with the diencephalon by the optic stalk.

a. The chorioid fissure. The invaginated portion of the vesicle is notched ventrally, the notch extending along the ventral surface of the optic stalk as a groove (chorioid fissure), through which the central artery of the retina reaches the optic cup.

(1) Closure of the fissure. It becomes a tube through approximation and fusion of its edges. Incomplete closure supposedly gives rise to absence of a sector of the iris, ciliary body or chorioid; this is known as coloboma.

(2) Obliteration of the optic stalk. This takes place upon growth of axons from the retina. The axons fill the lumen of the stalk on their way to the brain and the stalk is transformed into the optic nerve.

b. Pigment layer. The outer, thinner layer of the optic cup becomes this portion of the adult retina. Pigment appears very early.

c. Nervous layer. The internal, thicker layer of the cup becomes this retinal portion.

(1) Pars caeca. This is the zone bordering the rim, which later is subdivided into the pars ciliaris and pars iridica, respectively (pp. 97, 98).

(2) Pars optica (visualis). The more centrally located portion, separated from the former by the ora serrata. In it develop the rods and cones and the other layers of the adult retina (p. 98).

B. The lens. The ectoderm in front of the optic vesicle thickens to form the lens placode, which is soon changed into a vesicle.

1. Position. After invagination of the placode has ended the lens vesicle occupies the concavity of the optic cup.

2. Differentiation of the walls. From its early formation the lateral wall of the vesicle is thinner than the medial wall.

a. Lens epithelium. The cells of the lateral wall remain as low columnar elements and form this part of the lens.

b. Lens fibers. The cells of the medial wall become much elongated, their nuclei degenerate and they become the transparent long prisms or fibers.

c. Disappearance of the lens cavity. This is gradually obliterated when the rapidly elongating fibers come to be in contact with the posterior surface of the lens epithelium.

3. Capsule. It is apparently formed by the cells of the lens vesicle, but it lacks a definite structure.

C. The vitreous body. Fills the space between the lens and retina; it is produced by the latter, and is secondarily invaded by mesenchyme, some of which enters with the central artery.

1. The hyaloid artery. This is the branch of the central artery that crosses the developing vitreous body and spreads over the posterior surface of the lens. It degenerates, leaving the hyaloid canal (p. 100).

2. Pupillary membrane. This contains small vessels supplying the rest of the lens; they are derived from the peripheral rim of the chorioid.

D. The fibrous and vascular coats. They arise from mesenchyme which forms a double layer around the developing eye. The outer layer gives rise to the sclera and cornea, while the inner produces the iris, ciliary body and choroid. The anterior chamber arises as the result of degeneration of the mesenchyme between the lens and the surface ectoderm. Its continuous peripheral extension separates the iris from the cornea.

IV. The Ear

The formation of the external auditory meatus and the tympanic cavity has been already indicated (p. 124), as well as the origin of the ossicles of the middle ear (p. 150). Only the internal ear will be considered here. Its epithelial lining is of ectodermic origin.

A. The auditory placode. This is an area of thickened ectoderm located on each side of the hind-brain. The placodes appear very early, when a few somites are present in the embryo.

B. The otocyst, or auditory vesicle results from invagination of the placode. It loses all connection with the outside, but near the point where the otocyst joined the ectoderm there appears the:

1. Endolymphatic duct, which is a tubular outpocketing ending blindly distally; the blind end is the endolymphatic sac.

2. Division into regions. The otocyst elongates dorso-ventrally; its narrow ventral part becomes the:

a. Cochlear duct, which will soon coil to form the cochlea.

b. Vestibular portion. This is the more expanded, dorsal portion of the otocyst.

3. Subdivision of the vestibular portion. This will produce dorsally the:

a. Semicircular canals. The anterior and posterior arise from a single pouch at the dorsal border while the lateral begins as a horizontal outpocketing placed a little more ventrally.

b. Formation of the ampullae. The anterior and posterior semicircular canals have a common opening dorsally into the vestibule, but their opposite ends and the rostral end of the lateral canal are dilated into the ampullae (p. 102).

c. Utriculus and sacculus. They develop from the more ventral part of the vestibular portion through the formation of a constriction. The semicircular canals are attached to the utriculus, while the cochlea is connected with the sacculus.

d. Further development. The general shape of the inner ear of the adult is attained during the 3rd month. The utriculus and sacculus become separated from each other, but they keep their connection with the endolymphatic duct.

C. The bony labyrinth.

1. Production of cartilage. The mesenchyme surrounding the developing otocyst (membranous labyrinth) produces cartilage which completely encloses the labyrinth.

2. Formation of the perilymphatic space. Later on the cartilage next to the labyrinth undergoes regression and the space thus formed becomes the perilymphatic space.

3. Ossification. The bony labyrinth is produced during the 5 th month by replacement of the cartilage capsule by bone. The modiolus of the cochlea develops directly from mesenchyme as a membrane bone.

   Histology and Embryology 1941: Histology - 1 The Cell | 2 The Tissues | 3 The Organs     Embryology - 1 General Development | 2 Organogenesis | Bibliography

Cite this page: Hill, M.A. (2021, April 15) Embryology Histology and Embryology 1941 - Embryology 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Histology_and_Embryology_1941_-_Embryology_2

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© Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G