THE PALATE AND TONGUE. 555 



the inner surfaces of the maxillary arches. The palatal pro- 

 cesses grow rapidly, and by the beginning of the third month 

 the anterior ends of the maxillary processes, MX', have met and 

 fused with each other in the median plane, immediately behind 

 the premaxillary processes, or outgrowths from the processus 

 globulares, FO'. A small aperture is left in the median plane 

 between the four palatal processes, and persists as the foramen 

 incisivum. The completion of the palate is effected by the 

 extension backwards of the fusion of the inner edges of the 

 maxillary processes, towards their hinder ends. Occasionally the 

 union fails to take place properly, and the malformation known as 

 cleft palate results. 



By the formation of the palate, the anterior part of the mouth 

 cavity becomes divided into dorsal or nasal, and ventral or buccal 

 portions, and the communication between the posterior nostrils 

 and the buccal cavity is shifted backwards to the level of the 

 hinder edge of the palate. 



The septum narium is formed in the first instance by 

 upgrowths from the inner edges of the palatal processes, which 

 fuse together in the median plane, and grow dorsalwards as a 

 partition, dividing the nasal chamber into right and left halves. 



4. The Tongue. 



The tongue arises from the floor of the fore-gut, so that its 

 epithelial covering is entirely of hypoblastic origin. It is formed 

 from two rudiments, which are at first completely separate from 

 each other ; an anterior median swelling, the tuberculum impar, 

 from which the body and tip of the tongue are developed ; and 

 a posterior V-shaped ridge, which gives rise to the root of the 

 tongue. 



On the fifteenth day (Fig. 237) the ventral ends of the 

 mandibular arches, MN. almost meet each other in the median 

 plane ; the ventral ends of the hyoid arches, HY, are some 

 little distance from each other ; and the ventral ends of the 

 first and second branchial arches, BR.I, BE.2, are still further 

 apart. There is thus left in the floor of the pharynx, between 

 the ventral ends of the visceral arches, a triangular, meso- 

 branchial area, the apex of which is directed forwards. From 

 the dorsal surface of this area the tongue is developed ; while 

 the heart (Fig. 232) lies immediately beneath it. 



