180 H. SPENCER HARRISON. 



of an important difference between the dentary and the other 

 dentigerous bones as regards their tooth supply. 



Embryo 8, 19a. — This was a newly-liatched individual 

 and considerably more advanced than the last. In transverse 

 sections thi'ough the lower jaw the fully formed teeth are 

 seen to be firmly united with the jaw, and many have cut 

 the gum (fig. 14). The internal surface of the tooth is lined 

 by a layer of slightly flattened odontoblasts (fig. 15, od.). 

 The labio-dental and the internal dental grooves are now 

 formed, and the glandular structure is evident (fig. 14, 

 Z. d. gr., lin. gr., gl.). At the base of the internal groove is a 

 ridge of cells which is non-glandular in structure, and prob- 

 ably represents the persistent dental portion of the labial 

 ingrowth {r. d. I.). The crista gingivalis (c. g.) has also 

 become cut off by the formation of the internal fossa. At 

 the junction of tooth and bone the attachment of the inner 

 limb of the V-shaped sides of the labio-dental groove to the 

 calcified tissue is very intimate (fig. 15), and the same is the 

 case to a less extent with the corresponding limb of the 

 internal groove, although in the section (fig. 15) it had 

 broken away. There is a thin layer of mesoderm between 

 the epithelium and the bone, below the attachment of the 

 former, and this is concerned in the formation of new bone 

 lamellee at later stages. 



The teeth are at this stage typically acrodont, 

 and in the lower jaw their fusion in a longitudinal direction 

 can clearly be seen. 



The enamel has been completely removed by decalcifica- 

 tion in the tooth figured. 



Stage T and later. 



Stage T includes young individuals, from a few weeks to a 

 few months old, measuring 15 — 17 cm. in length. 



In a skull measuring 19 mm. from premaxilla to occipital 

 condyle, on the right side the condition of maxillary and 

 palatine teeth is much the same as in the skull of Stage S, 

 but the general size is somewhat larger. The chief interest 



