312 Veterinary Obstetrics 



these enlargements tend to atrophy and finally to completely dis- 

 appear. 



Each enamel organ soon consists of a flask-like Vesicle of epi- 

 blast with a narrow neck, which is still continuous with the epi- 

 thelium of the mouth by a cord- like constriction, while the 

 distal end, or fundus, of the flask is enlarged and spherical. 

 Beneath this enamel organ, there soon forms, from the meso- 

 bla.stic connective tissue, the dental papilla, which pushes up 

 into the sac, causing an invagination of its base. The enamel 

 sac now invests the top of the papilla as a cap and soon takes 

 on the form of the future tooth. The enamel organ is then in 

 the form of a flattened sac, the distal end invaginated, so that 

 the two walls are brought into close proximity. See Fig. 43. 



Upon the external surface of the dental papillae, odontoblasts 

 arise, from which the dentine of the tooth is formed. The 

 enamel prisms are formed from the epithelium of that part of 

 the walls of theenamel sac which lie in immediate contact with the 

 dental papilla, while the superficial wall, i. e. that portion con- 

 tinuous with the neck-like mass of epithelial cells still maintain- 

 ing connection with the mouth cavity, disappears without tak- 

 ing any recognized part in the formation of the enamel tissue. 

 The dentine, forming from the odontoblasts upon the apex and 

 sides of the papilla, soon invests these portions in the hard ivory 

 substance, while the base and center of the dental papilla con- 

 tinue as the tooth pulp. The base is at first wide open, but, as 

 the tooth develops, it gradually contracts and finally, in some 

 teeth, there grow out projections or septa of dentine, to consti- 

 tute the roots or fangs, which divide this cavity into two or 

 more distinct openings, according to the individual tooth or 

 species, and, through these apertures, the blood vessels and 

 nerves pass to the pulp of the tooth. See Fig. 44. 



As the enamel organ sinks down into the jaw, there occurs a 

 conden.sation of the surrounding mesoblastic connective tissue to 

 constitute a capsule, the tooth follicle, which closely invests the 

 enamel organ and papilla. When the bony tissue of the jaw 

 forms, the follicle serves as periosteum for the tooth and alveolus, 

 while, from its inner layers, the external tooth tissue, the 

 cement, is developed. The origin of the dental tissues is then : 

 I, the enamel from the invaginated epiblastic cells; 2, the pa- 

 pilla, vessels, nerves and dentinal tubules from the mesoblastic 



