1048 EMBRTOLOGT, 



bud increases, and at the same time is hollowed into glandular pouches. The 

 submaxillary gland is the first to appear ; according to Bischoff, it is entirely 

 formed in the foetus of a Cow only an inch in length. The parotid gland is the 

 last to be formed. 



2. Teeth. — These organs are developed in the interior of a cavity, named the 

 dental follicle or sac, by means of the elements of three germs— one belonging to 

 the ivory, another to the enamel, and a third to the cementum. 



Follicle. — The dental follicle is an oval cavity, with walls composed of two 

 layers ; the external is fibrous and complete ; the internal — soft or gelatinous — is 

 allied at the bottom to the ivori/ germ. The latter is a prominence which is 

 detached from the bottom of the follicle, and has the exact shape of the tooth. 

 Its structure comprises, in the centre, delicate connective tissue provided with 

 vessels and nerves, and on the surface a layer of elongated cells. At the summit 

 of the follicle, facing the ivory germ, is the enamel germ ; it is exactly applied 

 to the dental pulp, which it invests like a cap, and is composed of a small mass 

 of mucous connective tissue covered by a layer of cylindrical cells — the ada- 

 mantine tissue of Renaut — joined to the buccal epithelium by the guhernaculum 

 dentis. The cementum organ manifestly exists in the Foal, according to M. 

 Magitot. The base of the ivory germ has been found, but it disappears rapidly 

 after having performed its function. 



How are the different parts of the dental follicle developed ? When the 

 maxillary arches are formed, the alveolar borders show a ridge which enters 

 the embryonal tissue of the jaws. On the inferior border of this epithelial ridge 

 are formed buds (enamel organs), in niunber equal to that of the temporary teeth 

 — these are the primitive buds. 



While these buds — dependencies of the ectoderm — are developing towards 

 the interior of the jaw, there appear the dental bulbs (ivory organs), which arise 

 from embryonic tissue ; these grow outwards and bury themselves in the summit 

 of the enamel organs, so as to form a cap over them. Afterwards the wall of 

 the dental follicle is detached from the base of the bulb, rises, and envelops the 

 two germs in an oval sac. 



Magitot and Legros have remarked in the embryo of a Mare that : 1. At 

 the hundredth day, the enamel organs of the incisiors are distinct, and are detached 

 from the epithelial layer ; the follicles of the molars are in a slightly more advanced 

 state. 2. At one hundred and ninety days, the incisor follicles are closed ; the 

 molars are almost in the same state. 3. At two hundred days, the follicles have 

 attained their complete development — which precedes by some days the appear- 

 ance of the dentine cap ; the permanent incisor follicles are visible, but not yet 

 closed. 4. At two hundred and twenty days the temporary follicles are very 

 voluminous ; the dentine cap is already considerable ; and the two coronary and 

 radicular cement organs are in place and quite developed. 



Formation of the ivorg, enamel, and cementum. — The ivory and enamel are 

 developed by the modification of the elements situated at the surface of their germ. 



It has been shown that the germ of the ivory, or dental pulp, had exactly 

 the fonn of the future tooth ; consequently the ivory which arises from its 

 periphery offers the shape of this tooth. The ivorg (dentine) is constituted by 

 the cells of the germ, which elongate, send out prolongations — the dental fibres 

 — that ramify and anastomose, and by an intercellular substance wliich is im- 

 pregnated with calcareous matter, is moulded around these fibres, and forms the 

 dental canaliculi. 



