678 REPRODUCTION. 



of the mouth is consequently derived from the external blastodermic 

 layer, and is originally continuous with the epidermis ; while that of 

 the pharynx and oesophagus, like the rest of the intestinal epithelium, 

 is derived from the internal blastodermic layer. 



The completion of the parts about the mouth is accomplished by the 

 continuous development of the processes above described, which grow 

 together in such a way as to diminish the size of the original orifice, 



and to modify its form in various direc- 

 FlG - 226 - tions. (Fig. 226.) The process which 



grows downward in the median line from 

 the frontal region, is called the intermax- 

 illary process, because it intervenes be- 

 tween those forming the superior maxilla, 

 and contains, at a later period, the inter- 

 maxillary bones. In quadrupeds the in- 

 termaxillary bones, containing the upper 

 incisor teeth, remain distinct from those 

 of the superior maxilla, the line of demar- 



HUMAN EMBRYO, about one month cation between them being indicated by a 

 old; showing the growth of the suture. In man, as a rule, the maxillary 



frontal process downward, and , . , .,, , . , . , 



that of the superior and inferior and intermaxillary bones are consolidated 

 maxillary processes from the side, with each other, the only permanent 



poss" sL S r m suture being that on the median line, be- 



tween the right and left halves of the up- 



per jaw. According to Geoffrey Saint-Hilaire,* a line of suture some- 

 times remains between the intermaxillary and the superior maxillary 

 bones. 



The two inferior maxillary processes, unite with each other, making 

 the lower border of the cavity of the mouth, and form, by their union 

 upon the median line, the inferior maxilla. In quadrupeds the infe- 

 rior maxillary bones present a permanent median suture ; but in man 

 they are consolidated into a single piece during the first year after birth. 



As the intermaxillary process grows from above downward, it becomes 

 double at its lower extremity, and gives origin to lateral offshoots, 

 which curl round and enclose two circular orifices, the anterior nares 

 (Fig. 22*7) ; the offshoots themselves becoming the aloe nasi. Their 

 external border subsequently adheres to the superior maxillary process, 

 leaving only a curvilinear furrow at the side of the nose, to mark the 

 place of consolidation. In many quadrupeds, this furrow remains an 

 open fissure, extending outward and upward from the orifice of the 

 nostril. 



The mouth at this time is wide and gaping, owing to the incomplete 

 development of the upper and lower jaw and the comparative insuf- 

 ficiency of the lips and cheeks. The soft parts afterward increase in 

 growth, and thus gradually diminish the size of the orifice (Fig. 228). 



* Histoire des Anomalies de 1'Organization. Paris, 1832, tome i., p. 581. 



