THE THYROID, THYMUS, AND THE TEETH. 559 



maintains that the thymus of man, like that of other Vertebrates, 

 is developed from the hypoblastic lining of the pharynx ; His' 

 observations, on the other hand, support an epiblastic origin ; 

 the thymus, according to him, being formed from the epiblastic 

 walls of the sinus praBcervicalis, the deep fissure at the side of 

 the neck caused by the overlapping of the hinder visceral arches 

 by the more anterior ones (cf. Fig. 240, su). 



The thymus gradually shifts backwards towards the root of 

 the neck, extending along the pneumogastric nerve and carotid 

 artery almost as far as the heart. It attains a great size in later 

 foetal life, and continues to increase after birth up to about the 

 end of the second year, when it measures two inches or more in 

 leugth. 



7. The Salivary Glands. 



The salivary glands commence to form early in the second 

 month, and by the end of the month have attained a considerable 

 size. The ducts arise as grooves of the buccal epithelium, 

 which by fusion of their lips become tubes ; the glands them- 

 selves are, at first, solid outgrowths of epithelial cells, which 

 later become hollowed out by extension of the cavities of the 

 ducts into their substance. The submaxillary glands appear 

 first, then the parotid, and lastly the sublingual glands. 



8. The Teeth. 



The teeth are developed in man in very much the same way 

 as in the rabbit. In embryos about seven weeks old the epi- 

 thelium becomes thickened along the border of each jaw, and the 

 deeper or Malpighian layer of the epithelium grows down into the 

 substance of the jaw as a continuous keel-like ridge, the common 

 enamel germ. This soon becomes enlarged at intervals to form 

 the enamel organs of the milk or deciduous teeth, while between 

 the enamel organs the ridge becomes less conspicuous, and 

 ultimately disappears. 



Each enamel organ is flask-shaped, consisting of a terminal 

 enlarged portion, buried deeply in the jaw, and a narrow neck 

 or stalk which connects the enlarged part with the surface 

 epithelium of the jaw. Opposite each enamel organ the con- 

 nective tissue of the jaw becomes more compactly arranged to 

 form the dental papilla (cf. Fig. 156, tm). The dental papilla 

 soon becomes moulded into the shape of the future tooth, and 



