DEVELOPMENT OF THE PALATE 



147 



parallel to the first plane of section. Transverse sections through the snout may also be pre- 

 pared to show the positions of tongue and palatine folds before and after the fusion of the 

 latter (Fig. 151). 



In pig embryos of 20 to 25 mm. the jaws are close together and the mandible 

 usually rests against the breast. Shelf-like folds of the maxillae, the lateral pala- 

 tine processes, are separated by the tongue and are directed ventrad (Figs. 151 ^ 

 and 152 .4). The median nasal processes also give rise to a single heart-shaped 



Nasal septum 



Lateral palatine process 



Nasal septum 

 Lateral palatine process — A 



Mandible 



Turbinate anlage 



'- — Tongue 



Fig. 151. — Sections through the jaws of pig embryos to show development of the hard palate. X 8. 



A, 22 mm.; B, 34 rmn. 



structure, the median palatine process (Fig. 152). In embryos of 26 to 28 mm. 

 the mandible drops, owing to growth changes, and the tongue is withdrawn from 

 between the palatine processes (Fig. 151 5). With the withdrawal of the tongue 

 the palatine folds bend upward to the horizontal plane, approach each other and 

 fuse, thus cutting off the nasal passages from the primitive oral cavity (Fig. 152 5)- 

 The primitive choance (cf . Fig. 153), formed by rupture of the membrane separating 

 the olfactory pits from the oral cavity, now lead into the nasal passages, which in 

 turn communicate with the pharynx by secondary permanent choana. At the 



