30 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



from the body wall behind. Sagittal and coronal sections of the 

 primitive pharynx (Figs. 22 and 23) give a better idea of the 



dorsum sellae 



prim, pharynx 

 2nd cleft recess 

 3rd cleft recess 



max. proc. 

 osition of oral plate 

 stomodeum 



lung bud 



Fig. 22. — Showing the Primitive Pharynx of a 3rd week embryo in sagittal section, 

 bounded by the Visceral Arches. (After His.) 



arrangement and constitution of the visceral arches than can be 

 had from a surface view. They are developed round the most, 

 anterior part of the fore-gut which forms the lining membrane of 

 the primitive pharynx. The pharyngeal lining membrane, there- 

 fore, is the same as that of the alimentary canal from which spring 

 all the organs and glands of digestion and assimilation. 



The Visceral Clefts. — The epithelium or hypoblast, which 

 lines the primitive pharynx, covers the inner aspects of the 

 arches and passes outwards in the recesses between them and 

 there comes in contact with the epithelial covering of the 

 body (epiblast) which dips in to meet it. The membrane thus 

 formed by the union of the epiblast and hypoblast in the 

 recesses between the arches, may be named the " cleft mem- 

 brane." It is never ruptured nor disappears in the development 



