DEVELOPMENT OF THE DIGESTIVE TUBE 631 



to the proctodaeum (fig. 280B). At the 6- to 7-mm. stage, the stomach-liver 

 area begins to rotate toward the right as indicated in figure 296B. At about 

 7 to 9 mm., the stomach-fiver area is projected to the right and anteriad, while 

 the midgut and hindgut regions move toward the left (see arrows, fig. 296C). 

 At the stage of development when the larvae approximate 10 mm. in length, 

 the stomach and intestinal areas are arranged as in figure 296D. Through the 

 larval stages to the time of metamorphosis, the midgut or small intestinal area 

 becomes greatly extended and coiled as shown in figure 296E. At the time 

 of metamorphosis, the small intestine becomes greatly reduced in relative 

 length (fig. 296F). 



The chick embryo manifests similar gastrointestinal torsion. The duodenal 

 area of the intestine and the gizzard are pulled forward toward the liver, while 

 the small intestine becomes coiled and lies to a great extent in the umbilical 

 stalk, to be retracted later into the abdominal area. 



At the 10-mm. stage in the pig, the digestive tract consists of a simple 

 tubular structure as shown in figure 297 A (Lineback, '16). In this figure, 

 the pyloric-duodenal area is projected forward toward the liver, where the 

 pyloric-duodenal area eventually is tied to the liver on the right side of the 

 peritoneal cavity, with the result that the forming stomach lies transversely 

 across the upper part of the abdominal cavity. The cecal and large intestinal 

 areas are rotated around the small intestine (see arrow, fig. 297A), when 

 the latter lies herniated within the umbilical cord. In figure 297B is shown the 

 condition in the 24-mm. pig. It is to be observed that there is now a half rota- 

 tion of the large intestine around the small intestine, the latter being consider- 

 ably coiled, while in figure 297C a complete rotation of 360 degrees is shown. 



Aside from these rotational movements, extensive coiling of the gut tube 

 occurs, especially in the higher vertebrates. For example, the small intestine of 

 the frog becomes coiled extensively during the larval period (fig. 296E). Refer- 

 ence to figure 297D and E shows a similar coiling of the large intestine of 

 the pig. 



Rotational movements of the intestine in the human embryo also occur. For 

 example, in the human embryo of about 23 mm., a condition is present, com- 

 parable to that of the pig embryo of 24 mm., and the future large intestine 

 has been rotated 180 degrees around the small intestine as shown in figure 

 282F. Unlike the pig, however, a complete rotation of the gut is not effected. 

 Also, the large intestihe does not later form into a double coil as in the pig. 

 In the human embryo soon after the intestine is retracted from its herniated 

 position in the umbilical cord (fig. 282G), the cecal area of the large intestine 

 becomes fixed to the right side of the peritoneal cavity near the crest of the 

 ilium (Hunter, '28). The ascending, transverse, and descending portions of 

 the large intestine are then developed (fig. 364G, H). 



4) Histogenesis of the Intestine. During histogenesis of the intestine, two 



