320 



TEXT-BOOK OF EMBRYOLOGY. 



ectodermal origin. In the medial line of the roof of the oral cavity, near the 

 pharyngeal membrane, the epithelium (ectoderm) evaginates to form Rathke's 

 pocket. This comes in contact with an evagination from the floor of the brain 

 and with it forms the pituitary body. 



The further development of the mouth consists of an elaboration of the 

 structures which primarily bound the oral pit and the growth of certain new 

 structures such as the teeth and the tongue. The first branchial arch fuses with 

 its fellow of the opposite side in the midventral line to form the symphysis of 

 the lower jaws, giving rise also to the lower lip and chin region. As the naso- 

 frontal process continues to grow, two depressions appear on its ventral border, 



Pharynx 



Hypophysis — 



Yolk sac 



Belly stalk 

 Caudal gut 



Hind-gut 



Kidney bud * 

 Fig. 285. — Sagittal section of reconstruction of a human embryo of 5 mm. His, Kollmann, 



one on each side, a short distance from the medial line. These depressions are 

 the nasal pits which indicate the beginning of the external openings of the nasal 

 passages. The part between the nasal pits is destined to give rise to the nasal 

 septum and the medial part of the upper lip (Fig. 136). The primary oral 

 cavity is divided into the oral cavity proper and the nasal cavity by outgrowths 

 from the maxillary processes. From the medial side of each maxillary process 

 a plate-like structure grows across the primary oral cavity toward the medial 

 line (Fig. 178). These two plates, or palatine processes, meet and fuse with the 

 lower part of the nasal septum (Fig. 286) . (For further details of this fusion, see 

 page 152 and page 199). The palatine processes thus form the palate, or the 

 roof of the mouth, which separates the mouth cavity from the nasal cavity. The 

 palate does not extend far enough backward, however, to separate the posterior 



