THE EMBRYO. 



1199 



thinned and drawn out to form the primitive or common mesentery. The portion 

 of this mesentery which is attached to the greater curvature of the stomach is 

 named the mesogastrium, and the parts which suspend the colon and rectum are 

 respectively termed the mesocolon and mesorectum (Fig. 764). About the sixth 

 week a lateral diverticulum makes its appearance a short distance beyond the 

 vitello-intestinal duct, and indicates the future caecum or boundary between the 

 small and the large intestine. This csecal diverticulum has at first a uniform 

 calibre, but its blind extremity remains rudimentary and forms the vermiform 



Midbrain. 



Cerebellum. - 



Pharynrjenl septum.-- 



Pharynx.- 



Auditory pit.' 



Aortic bulb.- 

 Stomach. 



/Optic vesicle. 



StomodKum. 



Ventricle. 



Cloacal dilatation --I* 

 of hind-gut. \\ 



Allanloic stalk." 

 Umbilical vein. 



...Liver. 



,. Mid-gut and yolk 

 stalk. 



,Allantois. 

 Umbilical artery. 



FIG. 759. Human embryo, about fifteen days old. Brain and heart represented from right side; alimentary 

 canal and yolk-sac in mesial section. (After His.) 



appendix (Figs. 764, 765). Changes also take place in the position and direction 

 of the stomach. It falls over on to its right surface, which henceforth is directed 

 backward, while its original left surface looks forward ; further, its greater curva- 

 ture is drawn downward and to the left, away from the vertebral column, while its 

 lesser curvature is directed upward, and the commencement of the duodenum is 

 pushed over to the right side of the middle line. The mesogastrium, being attached 

 to the greater curvature, must necessarily follow its movements, and hence it 

 becomes greatly elongated and drawn outward from the vertebral column, and, like 

 the stomach, what was originally its right surface is now directed backward and its 

 left forward. In this way a pouch, the bursa omentalis, is formed behind the stomach ; 

 this pouch is the future lesser sac of the peritoneum, and it increases in size as 

 the alimentary tube undergoes further development ; the entrance to the pouch 

 constitutes the future foramen of Window (Figs. 761, 765, and 768). The re- 

 mainder of the canal becomes greatly increased in length, so that the tube is coiled 

 on itself, and this increase in length demands a corresponding increase in the width 

 of the intestinal attachment of the mesentery, so that it becomes plaited or folded. 

 At this stage the small and the large intestine are attached to the vertebral col- 

 umn by a common mesentery, the coils of the small intestine falling to the right 

 of the middle line, while the large intestine lies on the left side. 1 



1 Sometimes this condition persists throughout life, and it is then found that the duodenum does 

 not cross from the right to the left side of the vertebral column, but lies entirely on the right side of 

 the mesial plane, where it is continued into the jejunum ; the arteries to the small intestine (rami 

 intestini tenuis) also arise from the right instead of the left side of the superior mesenteric artery. 



