CHAPTER XII. 



THE DEVELOPMENT OF THE ALIMENTARY TUBE AND 

 APPENDED ORGANS. 



The embryonic disk, composed of the three germ layers, primarily lies flat 

 upon the yolk sac (see p. 139; also Fig. 82). A little later the axial portion of 

 the embryo is indicated by the primitive streak, the neural groove (subsequently 

 the neural tube), the notochord, and the primitive segments (Fig. 74). Then 

 along each side of the axial portion and at the cephalic and caudal ends, the 



Neural tube — Cm 



Oral fossa 



Heart 



_ Yolk sac 



Hind-gut 



Allantoic duct 



Belly stalk 



Fig. 282. — Lateral view of human embryo with 14 pairs of primitive segments (2.5 mm.). Kollmann. 



The yolk sac has been cut off. The fore-gut, mid-gut and hind-gut, as indicated in the figure, 



together constitute the primitive gut. Compare with Fig. 283. 



germ layers bend ventrally and medially and finally meet and fuse in the mid- 

 ventral line (p. 141). The portion of the entoderm ventral to the notochord is 

 bent into a tube which, for the most part, becomes pinched off from the parent 

 entoderm and is suspended in the embryonic ccelom by the common mesentery 

 (Figs. 141 and 142). This entodermal tube is the primitive gut. At first it is 

 but slightly elongated and is closed at both ends. On the ventral side, however, 



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