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 tw T o 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 



lias, tb 



Sept. 



max.tb. 



Pal. 



Jk.o. 



Ton. 



Mi. 



Fig. 140. — Pig, 20 mm. Transverse Series 59, Section 522. 

 Jk.o, Jakobson's organ, lat, Lateral ethmoid cartilage, max. id, Maxillo-turbinal fold. Afk, 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 



