1244 THE DIGESTIVE SYSTEM. 



The lower lip similarly is formed from the tissues covering the mandibular arches. 



By an ingrowth of epithelium from the surface of the frontal and maxillary processes 

 into the subjacent mesoderm and by subsequent desquamation of the superficial layers, a 

 groove is formed between the lips and cheeks, on the one hand, and the alveolar ridges on 

 the other. This groove when deepened forms the vestibule of the mouth, and is termed 

 the alveolo-labial sulcus, or labial groove. 



The surface covered with mucous membrane becomes everted, to form the red portion 

 of the lips, and at birth is divided into an outer smooth portion, and an inner portion 

 whose surface is villous, termed pars villosa. The distinction between these two parts 

 disappears shortly after birth. 



Several explanations of the formation of the philtrum or groove on the front of the 

 upper lip have been put forward ; most probably it is produced by the union of the margins 

 of the two processus globulares with one another, the floor of the groove being formed 

 by their line of union, and the ridges bounding the groove at the sides corresponding 

 to the medial portions of the globular processes. 



The parotid glands are formed as outgrowths of the epithelium in the outer wall of 

 the alveolo-labial sulcus. 



The outgrowth in each side has been found in embryos 8 mm. long. It is at first 

 a furrow. The posterior part of the furrow becomes closed off from the mouth cavity 

 and forms a tube, which grows backwards for some distance on the surface of the first 

 visceral arch. 



The terminal portion of the tube formed in this way gives rise to a number of buds, 

 which divide repeatedly, and form the lobules of the gland. 



These are at first solid, and the alveoli do not become hollowed until about the 

 twenty-second week. 



The epithelium of the terminal buds forms the secreting glandular epithelium, while 

 that of the stalk forms the lining epithelium of the duct. 



DEVELOPMENT OF THE TEETH. 



A tooth may be described as a calcified papilla of the mucous membrane, composed 

 of two chief parts namely, the substantia adamantina formed by the epithelial layer, 

 and the substantia eburnea by the connective tissue layer of the mucous membrane. 

 The details of the process by which such a tooth is developed from the two layers of the 

 mucous membrane are both numerous and intricate, and can be but briefly described here. 



In lower vertebrates (sharks, rays, etc.), teeth which correspond essentially, both 

 in structure and development, to those of mammals are found on the surface of the 

 body, and are known as dermal teeth. The following outline of the development of the 

 dermal tooth of a shark may assist in rendering the development of the human teeth 

 more intelligible : 



First, a papilla is formed from the corium or connective tissue layer of the skin 

 (Fig. 970, B), and this papilla is covered over by the epithelial layer. 



Next, the superficial (connective tissue) cells of the papilla begin to form a layer 

 of ivory on the surface of the papilla (Fig. 970, C), which it soon encases, the remains 

 of the papilla persisting in the interior as the future tooth pulp. At the same time the 

 deepest cells of the epithelium deposit a layer of adamant outside the ivory over the 

 summit of the papilla (Fig. 970, C), and subsequently the two adamant and ivory- 

 become inseparably united, thus giving rise to the substance of the tooth. 



At a later period the epithelium covering the summit disappears and the tooth comes 

 to the surface ; this constitutes its eruption (Fig. 970, D). 



In the case of the mammalian tooth a similar process takes place, not, however, on 

 the surface, but deep down in the substance of the gum, into which a downgrowth of 

 epithelium has previously taken place. This epithelial downgrowth spreads out in the 

 substance of the jaw, and into it the papilla grows up, and goes through the other changes 

 described above, as if the whole process took place on the surface. 



Development of Human Teeth. The following is a brief summary of the chief 

 events in the. development of a human tooth. For convenience in expression and terms, 

 the description refers to the development of a lower tooth. The upper teeth are, of 

 course, developed in a manner exactly similar. 



1. The first distinct evidence of the development of the teeth is to be found in a 

 thickening of the mouth epithelium, at the site of the future gum, and a resulting down- 

 growth of its deeper portion into the substance of the primitive jaw (Fig. 971, L). 

 epithelial downgrowth is continued along the whole length of the gum, and is known as 



