516 



EMBEfOLOGY. 



Fig. 290. — Gross section througli the head of an embryo Tig 3 cm 

 long, crown-rump measurement. 



The nasal cavities are seen to be in communication with the oral 

 cavity at the places designated by a * ; K, cartilage of the nasal 

 septum ; m, turbinal cartilage ; /, organ of Jacobson ; /', the 

 place where it opens into the nasal cavity ; gfj palatal process ; 

 of J maxillary process ; zl, dental ridge. 



The horizontal palatal processes of the upper jaw increase in size, 



and finally their 

 free edges en- 

 counter in the 

 median plane the 

 still broad nasal 

 septum, which has 

 grown down yet 

 farther into the 

 oral cavity. Then 

 the parts men- 

 tioned begin to 

 fuse with one an- 

 other from before 

 backward. 



Two stages of 

 this process are 

 illustrated by the 

 accompanying 

 figures (figs. 290, 



291), in which cross sections through the anterior end of two embryo 



.Pigs are repre- 

 sented. Figure 



290 shows the 



stage at which 



the palatal 



plate (gf) of 



the maxillary 



process (of) 



has advanced 



close to the 



lower margin 



of the nasal 



septum. Oral 



and nasal cavi- 

 ties are still 



in communica- 

 tion by means 



'of the very 



narrow palatal fissure indicated by an asterisk. 



In figure 291 the fusion has taken place. In this manner the 



Fig. 291. — Cross section through the head of an embryo Pig 5 cm. 



long, crown-rump measurement. 

 kf Cartilaginous nasal septum ; m, nasal turbinal process ; J, Jacobson^s 



organ with jfc, Jacobson's cartilage ; zl, dental ridge ; bl, covering 



bone. 



