176 H. SPENCER HABRISON. 



side of the teeth are the labial and lingual epidermal ingrowths 

 (fig. 8, man. lin., man. lah.), which are now broad bands of 

 cells projecting down into the mesoderm beyond the deepest 

 point reached by the enamel organs; in fact, as seen in 

 section, they embrace between them the edge of the dentary 

 as well as the base of the teeth. In the section figured 

 (fig. 8) the relations of the epidermal ingrowths are very 

 simple, but in some sections we may find anterior and poste- 

 rior portions of two enamel organs, the labial ingrowth 

 appearing as a process of one of them, and the lingual as a 

 process of the adjacent one. In the anterior and middle 

 regions of the jaw the enamel organs are mainly free from 

 the lingual ingrowth, but the posterior younger enamel 

 organs are found with an internal connection with it. 



Posteriorly the labial and lingual strands fuse, separate 

 from the epidermis, and pass backwards, as in Stage R, 

 into the space between coronoid and dentary. The fifteenth 

 mandibular tooth, the last at this stage, makes its first 

 appearance on this portion. 



Stage S. 



This stage embraces the later period of incubation. 



In a skull measuring 13*5 mm. from premaxilla to occipital 

 condyle the condition of the teeth is as follows (see also figs, 

 10 and 11) : 



The premaxillary teeth (fig. 10) are three in number on 

 each side, the third being considerably the largest. The 

 maxillary teeth (fig. 11) show a conspicuous alternation in 

 size, Nos. 1, 3, 5, 7, 9, on the left, being larger than the 

 intermediate teeth, although the first is smaller than the 

 others of the lai'ger series. On the right side the first three 

 are small, the middle one being the largest, while Nos. 4, 

 G, 8, 10, correspond to the larger teeth of the left side. We 

 see, then, that although the alternation is regular, there is a 

 tendency for the first two or three to be small, whichever 

 series they may belong to, and also that the two sides do not 

 correspond in this specimen. 



