Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 127 



mouth is bounded above by the fronto-nasal process (/. n. j>.), at 

 the outer edges of which are nasal grooves passing from the 

 nasal pits (n. p.) to the mouth (m.). The lateral edges of the 

 mouth lie between the somewhat divergent superior (s. m. p.) 

 and inferior maxillary processes (i. m. p.), the latter processes 

 forming also its lower boundary. As development proceeds, 



l^.v.cl. 



FIG. 40. THE MOUTH. 



A. Ventral view of head of chick embryo. B. Transverse section of 

 mouth (after Gegenbaur). 



a. Point where fronto-nasal process overarches the nasal groove, e. Inter- 

 nasal septum, fn. p. Fronto-nasal process, i. m. p. Inferior maxillary pro- 

 cess, m. Mouth, n. Nasal chambers, n. p. Nasal pit. p. Incomplete 

 palate, s. m. p. Superior maxillary process. 1st v. d. First visceral cleft. 



the fronto-nasal process overarches the nasal groove at the 

 point a, and fuses with the superior maxillary process, thus 

 converting the nasal groove at this point into a canal. From its 

 mode of formation it is clear that this canal at first opens 

 just within the primitive mouth ; but, by the forward growth 

 of the parts anterior to it to form the beak, and other changes, 

 its position becomes altered. In the fowl to some extent, but 

 more markedly in the rabbit, the stomodseal cavity becomes 

 divided into two chambers by the formation of a horizontal 

 septum developed from lateral outgrowths of the superior maxil- 

 lary processes (#>., Fig. 40, B., a diagrammatic cross section of 

 the mouth, in which the palatal processes, p., have not yet 

 met in the middle line). Anteriorly the upper or respiratory 



