THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 611 



Fig. 332. Roof of the oral cavity of a human embryo with 

 fundaments of the palatal processes, after His. Magnified 

 10 diameters. 



upper boundary to the mouth. In this way each olfactory pit with 

 its nasal groove is converted into a canal, which leads into the 

 oral cavity through an inner opening close behind the margin of 

 the upper jaw. The 

 membranous margins 

 of the upper and 

 lower jaws also lose 

 their superficial posi- 

 tions, because the 

 skin that covers them 

 is raised up into ex- 

 ternally projecting 

 folds, and forms the 

 lips, which from this 

 time forward consti- 

 tute the boundary of 

 the oral opening. 



A third stage, with 

 the development of the palate, practically completes the formation of 

 the face. (Compare pp. 515-17.) From the membranous upper jaw 

 there arise two ridges projecting into the mouth-cavity (fig. 290) ; 

 these become enlarged into the palatal plates, which grow horizontally. 



The plates meet in the median plane and fuse with each other and 

 with the median part of the frontal process, which has meantime 

 become reduced by the enlargement of the olfactory organ to the thin 

 nasal septum. Thus there is cat off from the primary oral cavity 

 an upper chamber, which contributes to the enlargement of the nasal 

 cavity, and which opens into the pharynx through the posterior 

 nares ; at the same time [as the result of this growth] there has 

 arisen a new roof of the mouth-cavity, the palate, which is after- 

 wards differentiated into hard and soft palate. 



A further differentiation of the face, which is now in the mem- 

 branous stage of development, is brought about by the process of 

 chondrification. This produces, however, in Mammals, as compared 

 with Selachians, only small and unimportant skeletal structures. 

 Some of these structures undergo degeneration (MECKEL'S cartilage), 

 some are utilised as auditory ossicles in the function of hearing, and 

 others are united to form the fundament of the hyoid bone. They 

 arise from the soft tissue of the first, second, and third visceral 

 arches ; in the case of the fourth and fifth arches there is not even 

 a process of chondrification in Mammals, so that with the closure of 



