1 66 



HISTOLOGY. 



these structures are rudimentary; if their closing plates ever rupture they 

 are soon restored so that permanent openings from the pharynx on the side 

 of the neck are not found. In mammals four clefts are indicated by ecto- 

 dermal depressions as shown in Fig. 187. Posterior to the mouth is the 



FIG. 186. HEAD OF A YOUNG DOG-FISH. 

 g. c., Gill cleft; m., mouth; n., nasal pit; sp., spiracle. 



FIG. 187. HEAD OF HUMAN EMBRYO OF 

 10 mm. 



c. s., cervical sinus; g. c. 2., second gill cleft; 

 h., hyoid arch; m., mouth; md., man- 

 dibular process; n., nasal pit; sp., audi- 

 tory (spiracular) groove. 



auditory (spiracular} groove, which is counted as the first' gill cleft; it gives 

 rise to the external auditory meatus and around it the auricle develops. 

 The ectodermal groove connected with the second gill cleft disappears. 

 Those of the third and fourth form a single deep depression on the side of 



the neck, called the cervical sinus, which 

 persists only in pathological cases, and is a 

 source of branchial cysts. 



The entodermal portion of the gill 

 clefts in a mammal is shown in Fig. 188. 

 The pharynx opens to the exterior at the 

 mouth, m, and divides posteriorly into the 

 trachea, tr, and oesophagus, oe. In the 

 median dorsal line it gives rise to the an- 

 terior lobe of the hypophysis, cut off at 

 a. I., and' in the median ventral line to the 

 thyreoid gland, t. The latter grows down 



through the hind part of the tongue, acquiring a position in front of the 

 trachea. Its branching terminal part becomes separated from its outlet, 

 the foramen caecum, by the obliteration of its duct (called the thyreoglossal 

 duct}. Thus the thyreoid gland is a detached clump of endothelial tubules 

 in front of the trachea. The entodermal portions of the gill clefts are four 

 paired lateral outpocketings. The first (i) extends to the auditory groove 

 in the ectoderm, and it becomes in the adult the auditory tube (Eustachian 

 tube) and the middle ear. It will be further described with the sense 

 organs. The second pharyngeal pouch (2) disappears except as it forms 

 a depression in the lower part of which the palatine tonsil develops. Its 



FIG. 188. DIAGRAM OF THE PHARYNX 



OF A MAMMALIAN EMBRYO. 

 a. 1., Anterior lobe of the hypophysis; f. c., 



foramen caecum; m., mouth; oe., 



oesophagus; p. b., postbranchial body; 



t., thyreoid; th., thymus; tr., trachea; 



i, 2, 3, 4, the pharyngeal pouches. 



