312 



STUDY OF PIG EMBRYOS. 



At this stage it consists of a large mass of imdifferentiated mesenchyma, covered 

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

 another until they meet in the median line below the nasal septum, Sept. As they 

 approach 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 resorption. 



nas.tb. 



Sept. 



max.tb. 



Pal. 



Ton. 



Or. 



Mk. 



FIG. 212. 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. 



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 ex- 

 tending 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. 213). The spinal 

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

 ject downward so as to enclose between them a distinct groove in the median ven- 

 tral line, which can be identified as the commencing anterior fissure of the cord. 

 In this groove runs a small, longitudinal blood-vessel, the arteria sulci, which from 



