THE THYEOID, THYMUS, AND THE TEETH. 5 



maintains that the thymus of man, like that of other Vertebrat' 

 is developed from the hypoblastic lining of the pharynx ; H 

 observations, on the other hand, support an epiblastic origi 

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

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

 the neck caused by the overlapping of the hinder visceral arc! 

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



The thymus gradually shifts backwards towards the root 

 the neck, extending along the pneumogastric nerve and carol 

 artery almost as far as the heart. It attains a great size in la1 

 fcBtal life, and continues to increase after birth up to about t 

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

 length. 



7. The Salivary Glands. 



The salivary glands commence to form early in the seco: 

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

 size. The ducts arise as grooves of the buccal epitheliu: 

 which by fusion of their lips become tubes ; the glands thei 

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

 later, become hollowed out by extension of the cavities of t 

 ducts into their substance. The submaxillary glands appe 

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



8. The Teeth. 



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

 as in the rabbit. In embryos about seven weeks old the ej 

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

 deeper or Malpighian layer of the epithelium grows down into t 

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

 enamel germ. This soon becomes enlarged at intervals to for 

 the enamel organs of the milk or deciduous teeth, while betwe( 

 the enamel organs the ridge becomes less conspicuous, ai 

 ultimately disappears. 



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

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

 or stalk which connects the enlarged part with the surfa 

 epithelium of the jaw. Opposite each enamel organ the co 

 nective tissue of the jaw becomes more compactly arranged 

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

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



