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 mesoyastrium. 



In many Fishes and Amphibia this condition is permanent. Even 

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



Fig. 102. 



Fig. ir.3. 



Fig. 162. Alimentary tube of a human embryo (R of His) 5 mm. long, neck measurement. From 



His, " Menschliche Embryonen." Magnified 20 diameters. 

 RT, RATHKE'S pocket; Uk, lower jaw ; SW, thyroid gland ; Ch, Chorda dorsalis ; Kk, entrance 



to larynx ; Lr/, lung ; My, stomach ; P, pancreas ; Lby, hepatic duct ; Ds, vitelline duct 



(stalk of the intestine); All, allantoic duct; IV, Wolffian duct, with budding kidney-duct 



(ureter) ; B, bursa pelvis. 



Fig. 163. Alimentary tube of a human embryo (Bl of His) 4 -25 mm. long, neck measurement. 



From His, " Menschliche Embryonen." Magnified 30 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, in order to find room for itself in the 

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



