358 TEXT-BOOK OF EMBRYOLOGY. 



an imperfect separation between the primitive gut and the anlage of the 

 respiratory system (p. 362). 



THE STOMACH. Occasionally the stomach is smaller than the normal. It 

 may even be a narrow tube resembling the other portions of the gut, owing to 

 lack of dilatation. Other congenital malformations, apart from transposition 

 (p. 355), are very rare. 



THE INTESTINES. One of the most common anomalies is the persistence of 

 the proximal end of the yolk stalk, forming Meckel's diverticulum (see p. 117). 

 This usually is attached to the ileum about three feet from the caecum. In ex- 

 ceptional cases it retains its lumen and, when the stump of the umbilical cord 

 disappears, forms a congenital umbilical fistula. Usually, however, the diver- 

 ticulum is shorter and ends blindly. Occasionally it becomes constricted from 

 the intestine and forms a cystic structure. (See also Chap. XIX.) 



Congenital stenosis and atresia may occur in different regions of the intestine, 

 the duodenum being the most common site. Normally the lumen of the 

 duodenum becomes closed for a brief period during development (p. 339), and 

 congenital closure of the lumen may represent a persistence of the early em- 

 bryonic condition. 



A conspicuous malformation is the persistence of the cloaca. The septum 

 which normally separates the latter structure into rectum and urogenital sinus 

 fails to develop, thus leaving a common cavity (see Figs. 361 and 362). In 

 addition to this the cloacal membrane may fail to rupture and the cloaca be- 

 come much distended. More often the septum develops in part, leaving only 

 a small opening between the rectum and urogenital sinus. After the latter 

 undergoes further development, the rectum comes to open into the urethra or 

 bladder, or into the vagina or uterus. 



Atresia of the anus is not infrequently met with. The cloacal (or anal) 

 membrane fails to rupture and the rectum ends blindly. In other cases the 

 rectum opens into the urogenital sinus, as described in the preceding paragraph. 

 Occasionally the lumen of the rectum is closed atresia recti and the gut ends 

 blindly some distance from the surface, being connected with the anal region by 

 a cord of fibrous tissue. 



Variations in the position of the intestinal loops, apart from transposition (p. 

 355), are of frequent occurrence. It is not customary to include these varia- 

 tions among malformations (see p. 340). The caecum (and appendix) and colon 

 present some striking variations. The caecum may be situated high up in the 

 abdominal cavity, the ascending colon being absent. Or it may be situated at 

 any intermediate point between that and its usual position in the right iliac 

 fossa. These variations are due to different degrees of development of the 

 ascending colon (p. 341). 



THE LIVER. Congenital malformations of the liver are rare. The most 



