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The portion of the wall of the primitive pharynx which lies between each pair of visceral arches and separates the clefts externally from the pouches internally is called the separating membrane. In the earliest stages it consists of ectoderm, mesoderm, and entoderm; then, for a time, the mesoderm disappears to re-appear again between the two epithelial strata at a still later period.

Round the margins of the dorsal part of the first pharyngeal or mandibular cleft are formed a series of tubercles which develop into the auricle of the external ear, and the cavity of the cleft becomes the external acoustic meatus (see p. 52). The first pharyngeal pouch and the adjacent part of the cavity of the primitive pharynx becomes the tympanic cavity and the auditory (O.T. Eustachian) tube. A part of the cavity of the second pharyngeal or hyoid pouch is represented in the adult by the supra-tonsillar recess, which lies in the side wall of the pharynx above the palatine tonsil (Fig. 56).

The third pharyngeal pouch opens like the first and second directly into the cavity of the fore-gut, but the fourth and fifth pouches lie in the lateral wall of a common recess which opens by a single aperture, the pharyngo-branchial duct, into the cavity of the primitive pharynx (Fig. 56).

The cavities of the third, fourth, and fifth pouches ultimately disappear, but before the disappearance takes place diverticula which, at first, are hollow but, afterwards, become solid are given off from the ventro-lateral parts of each, and solid epithelial outgrowths, the epithelial bodies, are formed from the dorso-lateral walls of the third and fourth pouches (Fig. 56).

The ventral diverticulum from the third pouch, on each side, forms the main part of the corresponding lobe of the thymus, and the ventral diverticulum of the fourth pouch either takes part in the formation of the thymus or it entirely disappears. The rudiment of the thymus is formed in the neck, but as the gland differentiates it extends and it migrates caudally, until its cephalic end lies near the caudal end of the thyreoid gland, at the level of the sixth ring of the trachea, and its caudal end is in the thorax at the level of the fourth costal cartilage.

The epithelial bodies derived from the third and fourth pharyngeal pouches form the structures known in the adult as the parathyreoid bodies. That derived from the third pouch migrates caudally more rapidly than its fellow formed from the fourth pouch; consequently the parathyreoid derived from the fourth pharyngeal pouch lies at the middle of the dorsal border of the corresponding lobe of the adult thyreoid gland, and the parathyreoid formed from the third pharyngeal pouch is situated at the caudal end of the corresponding lobe of the thyreoid gland and close to the cephalic end of the thymus.

The diverticulum formed from the ventral part of the fifth pharyngeal pouch is the ultimo-branchial body. After it separates from the pouch it becomes solid and is associated with the corresponding lobe of the thyreoid gland, but, apparently; in the human subject, it takes no part in the formation of that gland.

Derivatives of the Ventral Wall.-The diverticulum from the ventral wall of the primitive fore-gut, which is situated nearest the cephalic or anterior end of the gut, is the rudiment of the thyreoid gland. It commences in the median plane, between the ventral ends of the mandibular and hyoid arches, and grows ventrally, into the substance of the neck, then turns caudally, ventral to the cartilages which form in the second, third, and fourth arches, from which the hyoid bone and the cartilages of the larynx are developed. When the caudal end of the diverticulum reaches the region where the cephalic or anterior portion of the trachea will be formed it becomes bilobed, and thus is differentiated into the isthmus and the two lobes of the permanent gland. The stalk of the diverticulum, which extends from what becomes the oral part of the primitive pharynx to the isthmus of the gland, is the thyreoglossal duct. Its cephalic end remains as the foramen cæcum, which is situated in the dorsum of the tongue, at the junction of the ventral two-thirds with the dorsal third. The caudal end sometimes persists and is transformed into the third or pyramidal lobe of the thyreoid gland, which is attached to the dorsal border of the isthmus (Figs. 56, 61). The more caudally situated diverticulum from the ventral wall of the fore-gut is the rudiment of the respiratory system (Figs. 59, 60). When it first appears



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it has the form of a longitudinal groove bounded at its cranial end and laterally by an elevated ridge, named by His the furcula (Fig. 59). The caudal end of the groove soon dilates into a pouch, and then the pouch and groove are separated by a constriction, which passes from the caudal

towards the cranial end, from the more dorsal part of the fore-gut, which becomes the œsophagus. The constricting process ceases before the separation reaches the cranial extremity of the respiratory rudiment, which remains, therefore, in communication with the pharynx and forms the permanent laryngeal aperture. The tube formed by the separation of the groove is differentiated into the larynx and the trachea, and the caudal terminal dilatation soon divides into two lateral lobes, each of which is the rudiment of the epithelial lining bronchi and the lung of the corresponding side.

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The Tongue. The tongue is formed by four separate rudiments which lie in the ventral part of the cranial end of the primitive pharynx. Two of these are elevations formed on the caudal surfaces of the ventral ends of the mandibular arches,

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ABOUT FIVE WEEKS OLD (Diagrammatic).

The tongue is well formed, the trachea and oesophagus are separated, the bronchi have commenced to branch; the duodenal curve is well formed, and the cæcum has appeared in the loop of the mid-gut. The cloaca is partially separated into genito-urinary and rectal portions.

one on each side. The third is a median elevation, the tuberculum impar, which is situated immediately caudal to the conjoined ventral ends of the mandibular arches,

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and the fourth, called the copula, formed by the conjoined ventral ends of the second arches, is separated from the tuberculum impar by the orifice of the thyreoid rudiment (Fig. 61).

The two lateral elevations on the mandibular arches unite to form the greater part of the ventral or anterior two-thirds of the tongue, upon which all the papillæ

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FIG. 61. SCHEMA showing stages in the development of the tongue.

are developed. The tuberculum impar either disappears or it forms the median part of the anterior two-thirds of the organ. The posterior or dorsal third of the tongue, which lies in the ventral or anterior wall of the permanent pharynx, is formed from the copula of the second arches. It follows from what has been said

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FIG. 62. SCHEMA showing further stages in the development of the diverticula from the primitive gut and modifications of the mid-gut and the mid-gut regions. The heart is not shown. (After Mall, modified.)

that the commencement of the thyreoid rudiment, which persists in the adult as the foramen cæcum of the tongue, must lie at the junction of the dorsal third with the ventral two-thirds. In many cases it appears to lie in the dorsal end of the ventral two-thirds, a position which may be associated with the fact that, in some cases, the rudiment of the thyreoid passes through the substance of the tuberculum




impar and not from between the tuberculum impar and the ventral ends of the hyoid arches.

Derivative of the Dorsal Wall (Seessel's Pouch).-The dorsal diverticulum from the cranial end of the fore-gut, to which the above term is applied, enters the base of the occipital region of the primitive head. The ultimate fate of the pouch is unknown in the human subject, but it has been suggested that it is represented by a depression in the mucous membrane of the cranial part of the pharynx, close to the pharyngeal tonsil, which is known as the pharyngeal bursa.

The reader who has followed this description will have noted that from the cranial portion of the fore-gut are formed the caudal or inferior part of the mouth (with the exception of the lips, teeth, and gums), the pharynx, the thyreoid gland, the thymus, the parathyreoids, the respiratory organs, and the oesophagus. The more caudally situated portion of the fore-gut is differentiated into the stomach and the first and second parts of the duodenum.

The stomach is formed from the part of the fore-gut immediately adjacent to

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FIG. 63.-SCHEMA showing complete separation of cloaca into dorsal and ventral hernia of a portion of the gut through the umbilical orifice. Mall, modified.)

ventral parts and the temporary The heart is not shown. (After

the esophagus, and the duodenum from the more caudally placed portion, which is directly continuous with the mid-gut.

The Liver and Pancreas. When the embryo is about three weeks old and has attained a length of 2.5 mm. a ventral diverticulum appears in the ventral wall of the duodenal part of the fore-gut, and when the age of the embryo is about four weeks and its length increased to about 4 mm. a diverticulum is formed in the dorsal wall a little nearer the cranial end. The ventral pouch is the rudiment of the liver, the gall bladder, the bile-ducts, and a portion of the pancreas, and the remainder of the pancreas is formed from the dorsal diverticulum (Figs. 57, 62, 63). The Derivatives of the Mid-Gut.-The mid-gut is that part of the primitive alimentary tract which lies between the more definitely enclosed fore-gut and hind-gut, and it is in free communication with the yolk-sac by the vitello-intestinal duct. It is transformed into the greater part of the small intestine.

The Derivatives of the Hind-Gut. The parts formed from the hind-gut are:-(1) The terminal part of the ileum; (2) the whole of the large intestine, except a small portion of the anal canal; (3) the urachus, the urinary bladder, the urethra in the female, and the greater part of the urethra in the male.1

T. B. Johnston, Journ. of Anat., Oct. 1913; H. v. Berenberg-Gossler, Anat., Heft. 1913.

As development proceeds the mid-gut and the cephalic (anterior) part of the hind-gut form a U-shaped tube which possesses a cranial (anterior) and a caudal (posterior) limb, and a ventral extremity which is connected with the yolk-sac by a narrowed and elongated canal, the vitello-intestinal duct (Fig. 57).

Upon the caudal limb of the loop, about the middle of its dorso-ventral height, an enlargement appears which is the rudiment of the cæcum and vermiform process of the adult. After this rudiment has formed the caudal limb of the loop undergoes rotation, being carried first to the left, then cranially, and finally to the right. As it is carried to the right it crosses the cranial (later ventral) aspect of the cranial limb of the loop, and when the rotation is completed the regions of the jejunum and ileum, the cæcum, the ascending and the transverse colon are defined.

After the rotation has occurred the tubular intestine formed from the mid-gut and the anterior part of the hind-gut, undergoes rapid elongation and is thrown into a number of coils.

When the embryo has attained the length of 10 mm., and is a little over a month old, the greater portion of the coiled gut passes through the umbilical orifice into an expansion of the coelom formed in the proximal part of the umbilical cord (see p. 47) (Fig. 63), which has replaced the allantoic or body-stalk as the medium by which the embryo is attached to the chorion. The herniated coils remain in the root of the umbilical cord until the embryo is about 40 mm. long, and about ten weeks old, when they return to the abdomen, and the cœlomic space in the umbilical cord disappears.

The Derivatives of the Posterior Part of the Hind-Gut.-When the caudal portion of the hind-gut is first enclosed its terminal extremity and its ventral wall are bounded by the caudal portion of the primitive streak, which is bent ventrally during the folding-off of the embryo.

The terminal part of this portion of the gut becomes expanded, forming a chamber called the entodermal cloaca, into the ventral parts of which the ducts of the primitive kidneys, the pronephric or Wolffian ducts, open, one on each side.

The ventral part of the cephalic end of the cloaca is continuous with the allantoic diverticulum, and the dorsal part with a tubular portion of gut which forms the descending and possibly also the iliac and pelvic portions of the colon.

As the temporary tail is formed and projected first caudally and then ventrally, by the growth energy of the nodal point situated at the caudal end of the neural tube, a diverticulum of the caudal end of the dorsal part of the cloaca is prolonged into it, forming the tail gut. This soon becomes shut off from the cloaca. It entirely disappears before the temporary tail is absorbed into the caudal end of the body (Figs. 57, 62, 63).

At a later period the cloaca itself is separated into a dorsal part, the rectum, and a ventral part, the urino-genital chamber, by the formation of a septum, which commences in the angle between the allantoic diverticulum and the ventral wall of the cloaca, and is prolonged caudally till it reaches and fuses with the internal surface of the cloacal membrane, which thus becomes separated into urino-genital and anal portions, both of which disappear about the eighth week.

In both sexes the urino-genital section of the cloaca is separable into three parts: (1) a cranial part, which is converted into the urachus or middle umbilical ligament; (2) an intermediate part, which becomes the urinary bladder; and (3) a caudal part, which, in the female, is transformed into the urethra and the vestibule of the vagina, whilst in the male it is developed into the urethra.

Derivatives of the Stomatodæum.-When the stomatodæum is first definitely established, it is bounded cranially (anteriorly) by the caudal surface of the ventrally bent terminal part of the head, caudally by the conjoined ventral ends of the mandibular arches, and laterally by the dorsal parts of the mandibular arches, and the maxillary processes, which grow ventrally from the dorsal parts of the mandibular arches. The space is open ventrally, and it is closed dorsally by the bucco-pharyngeal membrane, which separates it from the fore-gut (Fig. 55).

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