268 



HISTORY OF THE HUMAN BODY 



Behind the mouth on each side there develops a row of out- 

 pushings, the pharyngeal pockets, which meet a corresponding 

 set of inpushings from the outside (Fig. 74). In fishes these 

 break through at the points of contact, and form the gill-slits, 

 a series of permanent openings (4-8 in number) that form a 

 communication between pharynx and exterior and. allow the 



FIG. 74. Pharynx and visceral arches in human embryo. [After His.] 



(a) Ventral aspect of early embryo, with the front part of the lower jaw and the 

 gill arches removed, (b) Inner view, looking ventrally, of the lower jaw and gill- 

 arches, corresponding to the part removed from (a), but taken from a somewhat 

 younger embryo. (c) Similar to (b), but taken from an older embryo, not far 

 from the age of (a). 



In (a) the most anterior of the arches sectioned is the mandibular ( = lower jaw), 

 succeeding which are the gill-arches in order, three being definitely formed. An- 

 terior to the sectioned arches are seen the two superior maxillary processes, which, 

 by their later union form the upper jaw. Between and a little above these is the 

 fronto-nasal process. In (b) and (c) the most anterior arch is the mandibular, with 

 the gill-arches succeeding it in order. The round median mass is the tuberculum im- 

 par, which, with the eminences immediately behind it, form the tongue. The thy- 

 reoid anlage and the glottis and epiglottis are seen in (c). 



escape of the water constantly taken in at the mouth and 

 used in respiration. One or more of these slits appear in 

 amphibian larvae and in a few forms persist throughout life, 

 but in reptiles, birds, and mammals, although the pharyngeal 

 pockets and their corresponding external depressions form 

 during embryonic life, but two or three ever break through and 



