486 PAUL E. LINEBACK 



In pig embryos of 24 mm. the torsion of the primary loop 

 has begun, so that the large intestine passes across the left side 

 of the duodenum and becomes the anterior limb of the loop (fig. 

 2). It extends from the freely-projecting caecum in a remark- 

 ably straight course, free from all convolutions, to the right- 

 angled bend where it descends to the rectum. The other hmb 

 of the loop, which forms the small intestine, has become three 

 times as long as the colic limb and is thrown into many convolu- 

 tions, arranged in linear series. Together they form a striking 

 and characteristic figure quite unlike anything seen in human 

 development. By referring to Mall's reconstructions of the 

 intestines of two 24-mm. human embryos (1897, Tafeln 21 u. 

 22) or to Johnson's more comparable drawing of a 22.8-mm. 

 specimen (Lewis, '12, p. 321) the differences will be apparent. 



According to Mall ('97) the convolutions of the human small 

 intestine are quite constant, and there are six w^hich are pri- 

 mary. With many secondary subdivisions, he found that these 

 could be identified in the adult. Following Mall, MacCallum 

 studied the development of the coils in the pig and likewise 

 found that in "embryos of the same size the coils are constant 

 in their arrangement and definite in their position." But this 

 conclusion ought not to be accepted without further investiga- 

 tions. In MacCallum's figures .of embryos of 23 and 25 mm., 

 there is a well-marked stretch of small intestine toward the apex 

 of the loop, which is quite free from coils. No such interval is 

 shown in figure 2, and having found it but once in many em- 

 bryos dissected, I must regard it as exceptional. The four 

 primary groups of MacCallum are not apparent in my speci- 

 mens, and the individual coils in figure 2 cannot be homologized 

 with those in MacCallum's reconstructions. 



The torsion of the primary intestinal loop is carried much 

 further in the pig than in man. The human intestine rotates 

 through an arc of approximately 180°, so that the original pos- 

 terior limb becomes anterior and vice versa. That is, it accom- 

 plishes such a rotation as is nearly completed in figure 2 and 

 stops at that point. But the pig's intestine goes further, per- 

 forming a complete revolution, as shown in figures 3 and 4. 



