The Teeth 317 



shown between Cand C in Fig. 46 B, and at D in Figs. 45 and 46 A, 

 before the time when the tooth comes into wear, and the dentinal 

 summit is worn away. In some cases, as shown in Fig. 48, this 

 fusion fails to occur and results in an opening into the pulp cavity,, 

 with food particles passing through between the laminae 

 into the pulp, causing a purulent inflammation of it, with 

 destruction of the tooth and other important complications. 

 Thus, in the early stages of embryonic life, aberrations in the 

 formation of the tooth germs serve in a variety of ways to in- 

 duce defects in these organs which lead to their early disease 

 and destruction, accompanied by an endless variety of com- 

 plications of more or less serious importance for the well-being of 

 the animal. 



The mammalian tooth, the hardest tissue in the body, is nor- 

 mally a product of epiblastic growth from the stomatodeum, 

 which has become invaginated into the bones of the maxilla and 

 mandible, but tooth tissues are not confined to these parts. 

 We have already related on page 301 that tooth tissue, histo- 

 logically and anatomically, is also formed in the squamous por- 

 tion of the temporal bone of the equine embryo, but here the 

 conditions closely simulate those obtaining in the oral cavity 

 itself, that is, epiblastic cells are invaginated into the deeper 

 layers of mesoblast. From the mesoblastic bone, the squamous 

 temporal is formed, while the incarcerated epi blast, as in the 

 jaws, develops into enamel tissue. 



From these considerations one might be led to state that the 

 invagination of epiblast into embryonic tissues which are later to 

 form bone leads to the development of teeth and that this condi- 

 tion is essential to their origin. This, however, is not wholly true. 

 Somewhat rarely in horses, and yet more infrequently in other 

 animals, we meet with dental substances in the ovaries and testi- 

 cles, and here the condition of the invagination of epiblast into 

 mesoblast, with the incarceration of the former in mesoblastic tis- 

 sue which is to ossify, does not occur. Possibly, here, the epiblast 

 becomes invaginated through the gubernaculum testis or the cor- 

 responding ligament of the ovary into the mesoblastic genital 

 gland, or it may reach the genital gland through the Wolffian 

 duct, which, according to some embryologists, arises by a longi- 

 tudinal invagination of the epiblast, and, while the tissues of the 



