652 



DEVELOPMENT OF THE ALIMENTARY CANAL 



Fig. 243. 



HEAD OF HUMAN EMBKYO, 

 at about the twentieth day. After 

 Longet ; from a specimen in the 

 collection of M. Coste. 1. Frontal 

 or intermaxillary process. 2. Pro- 

 cess of superior maxilla. 3. Pro- 

 cess of inferior maxilla. 



its lower extremity the intermaxillary bones, in which the incisor 

 teeth of the upper jaw are inserted. The next process (2) origin- 

 ates from the side of the opening, and, advancing toward the median 



line, forms, with its fellow of the opposite 

 side, the superior maxilla. The processes 

 of the remaining pair (3) also grow from 

 the side, and form, by their subsequent 

 union upon the median line, the inferior 

 maxilla. The inferior maxillary bone is 

 finally consolidated, in man, into a single 

 piece, but remains permanently divided, 

 in the lower animals, by a suture upon the 

 median line. 



As the frontal process grows from above 

 downward, it becomes double at its lower 

 extremity, and at the same time two off- 

 shoots show themselves upon its sides, 

 which curl round and inclose two circular 

 orifices, the opening of the anterior nares ; the offshoots themselves 

 becoming the alse nasi. (Fig. 244.) The mouth at this period is 

 very widely open, owing to the imperfect development of the upper 



and lower jaw, and the incomplete 

 formation of the lips and cheeks. 



The processes of the superior max- 

 illa continue their growth, but less 

 rapidly than those of the inferior ; so 

 that the two sides of the lower jaw 

 are already consolidated with each 

 other, while those of the upper jaw 

 are still separate. 



As the processes of the superior 

 maxilla continue to enlarge, they also 

 tend to unite with each other on the 

 median line, but are prevented from 

 doing so by the intermaxillary pro- 

 cesses which grow down between them. They then unite with the 

 intermaxillary processes, which have at the same time united with 

 each other, and the upper jaw and lip are thus completed. (Fig. 

 245.) The external edge of the ala nasi also adheres to the superior 

 maxillary process and unites with it, leaving only a curved crease 



Fig. 244. 



H E A D O F H L T M A X E M B R Y O at about 



the sixth week. From a specimen iu the 

 author's possession. 



