ANIMAL DENTISTRY. 47 



the eighth week of foetal life, when the deep celhilar layer 

 of the gingival cushion begins to grow inward into the sub- 

 stance of the jaw, which at that period is represented by 

 cartilage. The cartilaginous jaw at the same time grows 

 upward around the descending epithelium until it presents 

 a deep trough-like groove which the gingival cushion now 

 occupies. The outer or loose layer remains without and 

 persists through life as the gums, while the deep or dense 

 layer pushes deeper and deeper into the jaw, becomes better 

 organized to form the common enamel germ. The common 

 enamel germ which is no more than the deep epithelial layer 

 of the gingival cushion, is continued evenly through the 

 whole trough, but at the ninth or tenth week of foetal life 

 it becomes tumefied at regular intervals to form the first 

 real representatives of the future teeth (the enamel germs). 

 As the enamel germs devlop into larger bodies the sub- 

 stance between them gradually disappears, leaving only a 

 sniall mound for each temporary tooth. Simultaneously the 

 basement layer of the gingival cushion tumefies beneath 

 this mound and forces it upwards into a permanent body 

 (the dental papilla). The dental papilla consists, therefore, 

 of a body of connective tissue, capped with epithelium. 



At the third month of foetal life the epithelial layer un- 

 dergoes petrification to form the future enamel organ. The 

 connective tissue calcifies to form dentinal substance, while 

 the base of the papilla remains unchanged to form the pulp. 

 While this process is going on the papilla is gradually as- 

 suming the shape and form of a tooth crown, the first di- 

 vision of a tooth to form. The neck and fang occur by the 

 deposition of dentinal substance at the base of the papilla. 

 The aperture into the pulp cavity is at first a wide one, but 

 soon becomes narrow and takes its place at the end of the 

 fang as an apical foramen, the opening through which the 

 vessels and nerves enter the tooth. In the case of the molar 



