412 OUTLINES OF CHORDATE DEVELOPMENT 



oral sinus (stomodseum), the floor of which is still formed by 

 the imperforate oral membrane. Within the next few days this 

 membrane becomes perforated by the mouth opening. The 

 oral sinus is bordered posteriorly by the mandibular processes, 

 which do not quite meet in the mid-line, and antero-laterally 

 by the maxillary processes, which are widely separated medially, 

 the interval being occupied by the frontal process, a ridge over 

 the surface of the fore-brain. 



FIG. 171. Early stages in the development of the head and face. After 

 Rabl. A. Head of a human embryo of 8.3 mm., seen from in front (ventrally). 

 B. Head of human embryo of about 12 mm., seen from in front. For explanation 

 see text. 



An important advance is to be seen in the development of 

 the olfactory pits, which appear at the ends of the frontal 

 process. Bordering the olfactory pits are inner or medial 

 and outer or lateral elevations or olfactory processes (Fig. 171, 

 A). The maxillary and mandibular processes are now closer 

 together so that the opening of the oral sinus (now called the 

 mouth) becomes a transversely elongated slit. 



In the embryo of the fifth to sixth week (Fig. 171, B) the 

 olfactory pits have deepened and have moved in toward the 

 mid-line, thus separating the mouth from the fore-brain or 

 forehead region. At the same time the medial and lateral 

 olfactory processes become more prominent, and the former are 



