196 



STRUCTURE OF THE VERTEBRATES 



outside the peritoneum. Glands are usually suspended by long 

 mesenteries; the kidneys are rarely completely surrounded by 

 the peritoneum on the dorsal side; and the urinary bladder may 

 have but slight contact with the peritoneal lining. 



^erve cord 



Mesente 



Mesentery 



f-Intestine 



Muscle 



Peritoneum ^.^^^^ 



Peritoneum -^^ 



Fig:. B 



ndoderm 



Fig. C 



Gut 



Fig. 111. Development of the Mesenteries and Peritoneum. (A) Shows 

 the embryo in cross section showing the relationship between the perito- 

 neum, mesentery and smooth muscles of the gut. (B) The same structures 

 at a later stage, with glands developing in a retro-peritoneal position. The 

 extensions of the mesentery are shown in cross section (C) and in side 



view (D). 



When the mesentery develops in the embryo it is connected 

 ventrally as well as dorsally; but except for a few isolated re- 

 gions the ventral portion is soon lost. The dorsal mesentery (or 

 simply "mesentery") becomes very complex. The lengthening of 

 the intestine stretches it along the ventral margin so that it be- 



