546 



THE HUMAN EMBRYO. 



(cf. Fig. 215). The proctodaeal opening is not formed until a 

 much later stage, about the end of the fifth week, and is a 

 perforation of the integument rather than a distinct pit. 



During the fourth week the alimentary canal rapidly assumes 

 more definite form. The pharynx (Figs. 216 and 233) remains of 

 great width from side to side, and in connection with it the gill- 

 pouches, lungs, and other important structures are formed. The 

 oesophagus rapidly increases in length as the neck elongates ; and 



FIG. 233. Outline figure of the alimentary canal of a Human Embryo, 

 lettered by Professor His, Pr, and estimated as twenty-eight days old (cf. 

 Fig. 216). The figure is drawn from the right side, and the cavity of the 

 alimentary canal is alone represented, not the thickness of its walls. The 

 curved line bounding the figure on the left is the notochord. (From His.) 

 y 15. 



All. allantois. S, cloaca. Ds, yolk-stalk. Ep, epiglottis. Kt, laryngeal chamber. 

 Lbg, bile-duct. Lg, lunp. Mg, stomach. A', ureter. P, pancreas. RT, pituitary body. 

 Uk, mandibular arch. W, Wolffian duct. Zg, tongue. 



the stomach becomes a more conspicuous dilatation. The intestine 

 is long, narrow, and tubular ; it forms a prominent, ventrally 

 directed vitelline loop, from the apex of which the narrow yolk- 

 stalk arises, connecting the intestine with the yolk-sac. 



The alimentary canal is at first (Fig. 232) closely attached 



