296 



EMBRYOLOGY. 



the first three regions, the whole alimentary tube possesses a 

 mesentery (mesenterium), the part which is attached to the stomach 

 being designated by the special name mesogastriurn. 



In many Fishes and Amphibia this condition is permanent. Even 

 in the adult the alimentary tube takes only a slightly sinuous course 



Fig. 162. 



Pig. 163. 



rig. 162. — Alimentary tu1)e of a human embryo (R of His) 5 mm. long, neck measurement. From 

 His, " Menschliclie Embryonen." Magnified 20 diameters. 



RT, Rathke's poclcet ; Uk, lower jaw ; Sd, thyroid gland ; Ch, Chorda dorsalis ; Kk, entrance- 

 to larynx ; Lg, lung ; Mg, stomach ; P, pancreas ; Mg, hepatic duct ; Bs, vitelline duct 

 (stallc of the intestine) ; All^ allantoic duct ; JT, Wollfian duct, with budding kidney-duct 

 (ureter) ; B, bursa pelyis. 



Fig. 163. — Alimentary tube of a human embryo {Bl of His) 4-25 mm. lon^, neck measurement. 



From His, " Menschliche Embryonen," Magnified SO diameters. 

 The abbreviations mean the same as in fig. 162. 



through the body-cavity. The stomach appears as a spindle-shaped 

 enlargement of it. 



An alteration is brought about in all higher Vertebrates by 

 a more or less considerable increase in the length of the tube, 

 which eventually far exceeds that of . the trunk. Consequently 

 the alimentary tube, ia order to find room for itself in the- 

 body-cavity, is compelled to take a tortuous course. In this way 



