DEVELOPMENT OF THE ALIMENTARY TRACT. 



1697 



The digestive tube is at first closely bound to the posterior body-wall by a short, 

 broad mesoblastic band. This attachment, or primitive mesentery, from the lower 

 end of the oesophagus downward, gradually increases in its sagittal dimensions, at 

 the expense of its breadth, in consequence of the gut-tube leaving the dorsal wall and 

 assuming a more ventral position, the entire gastro-intestinal tube being thus attached 

 by a mesentery. That portion of the latter connected with the stomach is known as the 

 mesogastrium, that with the intestinal tube as the mesenterium commune (Fig. 1478). 



The elongation of the stomach soon results in loss of the primary sagittal direc- 

 tion of its axis, which becomes oblique, the lower end of the organ passing to the 

 right, while its upper end is displaced towards the left in consequence of the increasing 

 volume of the liver. Embryos of the sixth week exhibit marked change in the form 

 of the stomach, since the dorsal wall, later the greater curvature, has become bulged 

 spineward, while the ventral surface presents a slight concavity foreshadowing the later 

 smaller curvature. Somewhat later the stomach also undergoes rotation about its 

 longitudinal axis, its primary left surface becoming the ventral or anterior, and its 

 primary ventral border the lesser or upper curvature. The primary wall of the 



FIG. 1431. 



Allantoic duct 



External surface 



Cloaca 



Cloacal membrane 



_Tail-bud 



Part of caudal end of sagittal section of rabbit embryo of twelve days, showing cloacal space in communication 

 with lower end of gut-tube and allantoic duct. X 35- 



stomach consists of the entoblastic lining surrounded by the splanchnopleuric meso- 

 blast. The differentiation of the gastric glands begins towards the close of the third 

 month as minute epithelial outgrowths from the entoblastic layer. A few weeks later 

 the glands become branched, and the parietal cells appear as differentiations from 

 single epithelial elements lining the peptic follicles. In the fifth month the length of 

 the glands has increased to about .20 mm., and during the succeeding month to 

 from .40-. 70 mm. (Kolliker). Differentiation of the mesoblastic tissue into the inner 

 circular and outer muscular layers occurs during the fourth month. 



The lower funnel-shaped pyloric end of the stomach at first passes insensibly 

 into the relatively wide beginning of the characteristic U-shaped intestinal loop which 

 extends from the stomach ventrally, its closed end or arch being attached to the 

 vitelline duct, and then returns to the posterior body-wall to be continuous with the 

 terminal segment, which maintains its sagittal relations in close attachment with 

 the dorsal boundary of the body-cavity. The inferior limb of the loop early shows 

 beginning differentiation into large intestine, the junction of the latter with the small 

 intestine being indicated by the slight ceecal expansion. Even at this period a defi- 

 nite vascular relation has been established by the three main segments of the gastro- 



107 



