930 DR. A. HOPEWELL-SMITH AXD DB. H. W. MARETT TIMS ON 



trace of a tootli-germ. After an interval, another tooth {H) of 

 considerable size appears, lasting through three slides. It has 

 a dental papilla with blunted apex. As yet there is no definite 

 evidence of calcification. There are neither palatal nor labial 

 downgrowths of the dental lamina. This tooth we identify as 

 the first of the cheek series. 



The tooth-band continues backwards in a well-marked condition 

 giving evidence of a distinct swelling which in Stage II has 

 developed into an undoubted tooth-germ. Without a knowledge 

 of the subsequent history of this swelling, one would have 

 hesitated to attach importance to it. 



Posteiiorly to this is the large enamel organ of Stage I from the 

 neck of which springs a definite palatal downgrowth of the dental 

 lamina. The shape of the dental papilla is definitely molariform, 

 the outer cusp already attaining to a higher level than the inner. 



One more enamel organ (J), the most posterior in the jaw 

 at this stage, is to be seen following closely upon the preceding 

 one. It has but reached the flask-shaped stage. 



Thus it is seen that there are representatives of eight teeth in the 

 upper jaw at this stage which, by comparison with those present 

 in Stage II, we identify as incisors 1, 2, 3, 4 and 6, and pi-emolars 

 1, 3 and 4. The reasons which have led us to arrive at this con- 

 clusion will be detailed subsequently. 



LoivQT Jaw. — Near to the mandibular symphysis is a well- 

 defined involution of the oral epithelium into the subjacent 

 mesoblastic tissues. The cells, both peripheral and central, 

 of this tubular downgrowth have precisely the same characters 

 as those covering the alveolar margin. From the appearance of 

 these cells and from the inclination of the downgrowth (down- 

 wards and towards the middle line) being the same as that of 

 undoubted tooth-germs situated more posteriorly in the jaAv, the 

 opinion that it is a vestigial tooth-germ (a) seems to be justified. 

 Very closely following upon this structure and placed at a deeper 

 level in the jaw, is a small well-calcified tooth (6) showing both 

 dentine and enamel. The appearance of this tooth suggests 

 degeneration, and it may safely be asserted that this tooth, though 

 calcified, is not the one which will ultimately become the functional 

 incisor. 



The difiiculty in identifying the exact relationships of tooth- 

 germs close to the symphysis is always great, due in part to a 

 certain amount of crowding, accentuated in the Macropodidse by 

 the very large size of the functional incisor and in part to the 

 difficulty of coiTect orientation. We at first thought that these 

 two structures stood to each other in the morphological relation- 

 ship of predecessor and successor. From a careful consideration 

 of all the facts we believe that this is not the case, but that they 

 are the representatives of two separate teeth and that the close 

 approximation to each other is due to the crowding caused by the 

 large functional incisor. The appearance of a suggests a tooth 



