THE LARGE INTESTINE OF THE PIG 485 



an imaginary axis and forms the colon-labyrinth, later shaped 

 like a bee-hive, consisting of 3^ concentric outer and 3^ excentric 

 inner convolutions." 



In 1901 MacCallum described the development of the pig's 

 intestine, intending to show that its coils are measurably con- 

 stant. In a 32-mm. embryo, he states that the large intestine, 

 ''in the region where it turns to form the rectum" is thrown into 

 'irregular twists.' The complexity of this rectal group of coils 

 is said to increase in later stages, but its further evolution is not 

 described in detail. However, it is clear that MacCallum failed 

 to find a primitive loop, produced from an otherwise straight 

 colon, which elongated and grew into a helicoid spiral, as de- 

 scribed by Martin and Bonnet; and in the following study it 

 will be shown that the course of development is more compli- 

 cated than these authors have represented. 



Beginning with an embryo of 12 mm., it will be found, upon 

 dissection, that the intestine has formed its primary loop ex- 

 tending into the umbilical cord, and that torsion has not yet 

 occurred. The intestine at this stage may therefore be com- 

 pared with that of human embryos of 7-10 mm. In both forms, 

 the large intestine occupies the greater portion of the posterior 

 hmb of the loop, beginning at the small bulbus coli, which marks 

 the future caecum (fig. 1). The apex of the loop in the pig is 

 more persistently attached to the vitelline duct than in man, 

 and this duct has been cut across in figures 1 to 4. Moreover 

 the length of the primary loop in the pig is greater than in 

 human embryos. In the pig the distance from the base of the 

 loop to its point of attachment to the yolk-stalk is between 1 and 

 i of the length of the entire embryo. (For example, in figure 

 1 it is J, and in the reconstruction of a 12-mm. pig by Lewis it 

 is I). In human embryos possessing a primary unrotated loop, 

 as reconstructed by His, Elze, and Lewis, the length of the loop 

 is between 7 and 1^ of that of the whole embryo. It is not im- 

 probable that the distinctly longer and more slender loop in the 

 pig provides for the more extensive intestinal convolutions, char- 

 acteristic of the pig in the stages immediately following. 



