EMBRYO OF 20 MM. 



241 



a layer of epithelium. The two palate shelves continue to grow toward one an- 

 other until they meet in the median line below the nasal septum. As they ap- 

 proach one another the tongue descends. Ultimately the two palate shelves 

 unite with one another and with the overlying nasal septum. The epithelium of 

 the two shelves concresces and forms for a time a partition, which marks the 

 point of union of the two shelves, both with one another and with the nasal 

 septum. This partition persists for a short time only, for it soon disappears by 



i nas.tb. 



Sept. 



tuax.tb. 



Jk.o. 



Ton. 



Pal. - 



Mk. 



FIG. 140. PIG, 20 MM. TRANSVERSE SERIES 59, SECTION 522. 



Jk.o, Jakobson's organ, lat, Lateral ethmoid cartilage, max.tb, Maxillo-turbinal fold. Mk, Meckel's cartilage. 

 nas.tb, Naso-turbinal fold. Or, Oral cavity. Pal, Palate shelf. Sept, Cartilage of nasal septum. Ton, 

 Tongue. X 22 diams. 



resorption. The union of the palate shelves separates definitely the nasal and 

 oral cavities from one another. Their union is gradual, beginning in front 

 and gradually extending backward. It is a not infrequent anomaly that the 

 palate shelves fail to unite perfectly. When this occurs, there results the 

 condition known as cleft palate. 



Transverse Section through the Lower Part of the Neck (Fig. 141). The spinal 

 cord, Sp.c, shows a very great enlargement of the ventral zones, which now pro- 



16 



