44 HUMAN EMBEYOLOGY. 



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. 



Bound 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, after- 

 wards, 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 caecum, 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 



