490 PAUL E. LINEBACK 



In the following description the terms proximal or outer limb 

 will be applied to the part of the colon leading from the caecum 

 to the apex, and distal or irmer limb to the part from the apex 

 to the splenic flexure. The inner hmb in the adult is of much 

 smaller diameter than the outer, except toward the apex; and 

 its revolutions, which closely accompany those of the outer limb, 

 are hidden within the dome-shaped mass. Having in mind 

 these relations and the descriptions of previous writers that a 

 primary loop, with its apex at the middle point, simply winds up 

 to produce the adult form, the conditions in the embryo may 

 be carefully examined. 



At 50 mm. (fig. 5), as in' the adult, the colon may be divided 

 into two nearly equal parts. The proxitmal half (in fig. 5 and in 

 the following drawings) has been heavily stippled to contrast 

 with the distal half. Beginning at the caecum, which at this 

 stage points ventrally, the colon passes toward the dorsal body- 

 wall, near which it makes a rather sharp turn and doubles back 

 upon itself. After running ventrally it redoubles by a sharp 

 turn and goes dorsally. This folding continues back and forth 

 throughout the proximal half. Distally the colon consists of 

 several short coils, irregularly arranged, w^hich become adher- 

 ent to the body-wall near the duodenum. Except at this fixed 

 point, the colon at this stage is freely movable. 



The most notable feature of the followdng stage (fig. 6, from 

 an embryo of 55 mm.) is the elongation of the first loop in the 

 proximal half of the colon. The proximal or outer half of this 

 first loop is now clearly a portion of the basal convolution of 

 the permanent spiral. In figures 7 and 8 (from embryos of 64 

 and 75 mm. respectively) the first coil has further elongated, 

 accomplishing, in figure 8, one half of a revolution. In con- 

 nection with this development, the caecum has shifted toward 

 the left of the bod}'- where it becomes permanently located, 

 and the entire colon has become twisted upon itself, dupli- 

 cating the torsion of the primary intestinal loop of earher stages. 

 In other words, the part of the colon toward the caecum has 

 come to cross the left side of the distal part of the colon, just as, 

 in the 24-mm. stage (fig. 2), the large intestine crosses the left 



