DEVELOPMENT OF THE ALIMENTARY CANAL. 677 



mucous membrane of the alimentary canal, with its covering of invol- 

 untary muscular fibres. The separation of these two laminaB leaves 

 between them the peritoneal cavity. 



But in the anterior part of the body of the embryo, this separation 

 does not take place. Consequently, that portion of the alimentary 

 canal, namely, the oesophagus, remains in contact with the surround- 

 ing parts; and its anterior rounded extremity, the pharynx (Fig. 

 224, d), lies within the head, covered by the united tissues of. the 

 middle and external blastodermic layers. 



At this time there are formed, on the sides and front of the neck, 

 four nearly transverse fissures, the cervical fissures, leading from the 

 exterior into the cavity of the pharynx. These fissures are analogous 

 to the permanent openings at the sides of the neck in fishes, where 

 the gills ar located. But' in the mammalian embryo they have only 

 a temporary existence. The three lower fissures disappear entirely by 

 the subsequent adhesion of their edges ; and in the chick, according to 

 Foster and Balfour, are completely closed by the seventh day of incu- 

 bation. The upper fissure is converted into a narrow canal, leading 

 into the )harynx, but closed about its middle by a transverse partition. 

 The outer portion of this canal becomes the external auditory meatus ; 

 the inner portion, the Eustachian tube. The transverse partition is the 

 membrana tympani. 



In the mammalian and human embryo, the bands of solid tissue 

 between the cervical fissures are connected with the formation of the 

 mouth and face. By their increase in growth they become more or less 

 prominent folds, known by the name of the " visceral folds." The first 

 visceral fold grows rapidly forward, and divides into two somewhat 

 diverging processes, which approach each other from the right and left 

 sides toward the median line. The lower pair unite, and form the 

 inferior maxilla. The upper pair, which form the superior maxilla, 

 unite, not with each other, but with an intervening process which 

 grows from above downward, in the median line, between them. 



By the continued growth of these processes, above and below, about 

 the median line, there is included between them a depressed space, lined 

 with a' continuation of the integument, and situated immediately in 

 front of the extremity of the pharynx. This excavation is the cavity 

 of the mouth, enclosed on each side by the superior and inferior maxillae, 

 widely open in front, but terminating within by a blind pit, having as 

 yet no communication with the alimentary canal. 



Subsequently, an opening is formed between the back part of the 

 mouth and the cavity of the pharynx, by a perforation through the 

 blastodermic tissues at that point. This perforation takes place in the 

 human embryo, according to Burdach,* during the sixth week. The 

 opening thus formed marks the situation of the fauces ; and the ali- 

 mentary canal then communicates with the exterior. The epithelium 



* TraitS de Physiologie. Paris, 1838, tome iii., p. 468. 



