344 THE CAT. [CHAP. x. 



as the crown of the tooth is completed its base becomes constricted, 

 and then, as it grows, the papilla becomes narrower and so the fang 

 is formed, its soft substance becoming smaller and smaller through 

 the progress of calcification around it, till nothing but a minute 

 central pulp cavity is left, through which the vessels and nerves 

 pass. If the tooth be a molar, a separate cap of dentine is formed 

 at first for each cusp, while when the crown has been formed, the 

 development of the papilla becomes modified by the cessation of its 

 growth in certain parts (with corresponding ingrowths from the 

 alveolus), so that it becomes subdivided into as many parts as the 

 tooth is to have roots. The soft tissue left is the foundation of the 

 future roots, the pulp of the tooth now adhering only by them 

 instead of by one undivided soft body, as is the case where there is 

 to be but one root. 



From the epithelial sac of each growing first (or milk) tooth, a 

 little diverticulum is given off, containing a continuation of the 

 primitively inflected epithelium, and against this, another small 

 papilla rises and imbeds itself, and thus the foundation of each 

 second or permanent tooth is laid. The relations of the milk and 

 permanent teeth have been already given in the second chapter. 



As the teeth are formed, so also are the bony walls of the alveoli 

 which grow up and come to surround and embrace them. 



The formation of the dentine of the teeth takes place by 

 gradual transformation of the surface of the dental papilla (consist- 

 ing of round nucleated cells in a clear matrix) in such a way as to 

 leave the dental tubuli open, as already described. 



The enamel is formed by more complete transformation into 

 mineral substance of the epithelial enamel organ, while the cement 

 is formed by the ossification of the connective tissue immediately 

 surrounding the dental papilla, and leaving not only tubuli, but as 

 we have already seen, corpuscles similar in form and nature to the 

 corpuscles of true osseous tissue. 



The LIVER arises as two diverticula given off from each side of 

 the alimentary canal, immediately behind the stomach (Fig. 155, A), 

 each diverticulum consisting of both mesoblast and hypoblast. The 

 former thickens greatly, and becomes very vascular, while canals 

 from the two primitive diverticula extend and ramify within the 

 cylinders of anastomosing cells which it builds up. The growth of 

 this organ is extremely rapid. At first the liver is almost quite 

 symmetrical in form, its right and left lobes being nearly equal. 

 The gall-bladder arises as a special diverticulum of both hypoblast 

 and mesoblast. 



The PANCREAS appears as a bud from, the left side of the duo- 

 denum, where both hypoblast and mesoblast are thickened ; canals 

 lined with the former ultimately being developed within the latter. 



The SALIVARY GLANDS arise in a similar manner at the anterior 

 end of the elementary tube. They differ from the pancreas, however, 

 inasmuch as their cavities are lined with epiblast, since this it is 

 which forms the lining of the buccal invagination. 



