THE PHARYNX. 



101 



The shallow and widely open stomodaeum soon deepens and is now seen 

 to be specially bounded by certain prominences placed above, below, and at the 

 sides, within and from which the several parts of the face are eventually pro- 

 duced (figs. Ill, 119). These prominences are the fronto-nasal which projects 

 over the stomodaaum, and is formed primarily by the prominence of the fore- 

 brain, but afterwards acquires a considerable thickening of mesoblast, which ex- 

 tends into it from the basis cranii ; the mandibular or first visceral arch, which 



Fig. 119. PROFILE VIEW OF A HUMAN EMBRYO OF 



ABOUT THREE WEEKS, SHOWING ALL THE 

 CEPHALIC VISCERAL ARCHES AND CLEFTS. 



mx, maxillary process ; mn, raantlibular arch ; 

 d.C, duct of Cuvier ; jv, jugular vein ; c.r, cardinal 

 vein ; v.v, vitelline vein ; u.v, umbilical vein ; u.a, 

 umbilical artery ; all, allantois ; pi, placental attach- 

 ment of allantoic stalk ; off, olfactory depression ; 

 ot, otic vesicle. 



after passing obliquely round the fore- 

 gut, takes a horizontal direction on its 

 ventral side, and meets its fellow in the 

 middle line, the two together forming the 

 ventral boundary of the stomodaeum ; and 

 the maxillary process, which grows from 

 the base of the mandibular arch, and 

 projects on either side of the stomodaaum, 

 filling up the gap between the fronto- 

 nasal process and the mandibular arch, 

 and forming the lateral boundary. 



The separation of the stomodaeum into an 

 upper or olfactory and respiratory part and a 

 lower permanent buccal cavity, together with 

 the changes which occur in the fronto-nasal 

 and maxillary processes to produce the result, has 

 already been referred to in describing the de- 

 velopment of the nose (pp. 95 to 97). 



Pharynx. The remainder of the ali- 

 mentary canal below the mouth is nearly 

 simple, at first consisting, as before men- 

 tioned, of a tubular portion in front the 

 fore-gut ; a shorter tubular portion behind 

 the hind-gut ; and a middle part which 

 is freely open to the yolk (fig. 117). 

 The hind-gut remains simple throughout, except that the allantois grows out 

 from its ventral aspect. x But in front a differentiation soon makes its appear- 

 ance, the cephalic part becoming enlarged to form the pharynx, while almost 

 immediately behind this another enlargement forms the stomach. The hypoblast 

 lining the cavity of the pharynx grows out on either side successively at four distinct 

 levels, and to a less extent the epiblast dips in opposite the hypoblastic outgrowths. 

 In this way eventually four deep clefts between the pharynx and the exterior become 

 formed ; these are known as the cephalic visceral clefts (figs. 110, 119, in external 



1 In the human embryo the allantois appears to be formed by a direct continuation of the lateral 

 folds, which have united to form the main alimentary tube, while the part of the tube behind the 

 allantois (bursa) appears as a blind protrusion (His). 



