Fig. 110.—The same view, from another specimen.

Fig. 111.—Schematic representation of the development of the mesentery of the umbilical loop.

Fig. 112.—Alimentary canal, isolated and in section, of Echelus conger, the conger eel. (Columbia University Museum, No. 1812.)   Fig. 113.—Chelydra serpentina, snapping turtle; intestinal canal, pancreas, and spleen, isolated. (Columbia University Museum, No. 1369)

From the ileo-colic junction the large intestine of the turtle continues caudad to the cloaca in a nearly straight line. The same primitive condition of the intestinal canal may be observed in some members of man’s own class, the mammalia—as in certain edentates. Figs. 107 and [108] show the entire abdominal portion of the alimentary tract in Tamandua bivittata, the little ant-eater of Brazil. The stomach is turned cephalad and the great omentum elevated. The intestines are turned over to the right side.

It will be observed that in spite of the numerous coils of the small intestine the general arrangement of the alimentary canal corresponds to the primitive scheme shown in [Fig. 98]. The entire intestinal canal is attached by a continuous vertical mesentery to the dorsal median line of the abdominal cavity ventrad of the vertebral column and aorta. The growth in length of the small intestine has necessitated a corresponding lengthening of the attached border of the mesentery—consequently the membrane presents a pleated or crenated appearance. The cæcum is well developed, the ileo-cæcal junction being situated within the returning limb of the loop, a little distance from the apex.

In [Figs. 109] and [110], taken from the same specimens, the entire mass of the small intestines has been turned to the left so as to exhibit the right leaf of the common dorsal mesentery and the mesoduodenum, the latter containing the head of the pancreas. It will be noted that the mesentery, expanding beyond the duodeno-colic isthmus, is common to the small and to the proximal portion of the large intestine, i. e., to those segments of the alimentary canal which are developed from the two limbs of the umbilical loop. Figs. 107-[110] should be studied and compared together, as each supplements the others.

It will be observed, in reference to the change from the primitive loop to the subsequent increase in the length of the tube and the resulting arrangement of the mesentery, that three successive stages are to be considered, represented schematically in [Fig. 111]. In the earliest stage ([Fig. 111], I.) the two segments of the loop are of equal length, parallel to one another, the distance between the beginning and termination of the loop (1-2) being maintained throughout its extent. Hence the mesentery is of equal width in all its parts within the loop, only drawn out, i. e., away from the vertebral column, in accordance with the length of the loop. In the next stage ([Fig. 111], II.) the increase in the length of the intestine is accompanied by a corresponding widening of the mesentery. The points 1 and 2 are still approximately the same distance apart as in the earlier stage, but the increase in the length of the tube between these points forces the two limbs of the loop to abandon their early parallel course, and to form curved lines with the concavity turned toward the mesenteric attachment. In this condition the mesentery consequently forms a widely expanded membrane framed by the intestine and narrowing between the points 1 and 2 to a neck or isthmus which effects the transition between the expanded segment surrounded by the intestine and the rest of the dorsal primitive mesentery. Finally in the stage represented in [Fig. 111], III., the increase in the length of the small intestine has reached a point where a single curve is no longer sufficient for the accommodation of the growth. Consequently the tube now appears coiled and convoluted, and the mesentery, as it is attached to the gut, of necessity follows all the twists and appears fluted or pleated in its distal attached portion.

If we now carefully examine the conditions presented by the intestine and mesentery in a form like Tamandua (Figs. 107 and 108) we will find that they correspond to the developmental facts thus far considered. The termination of the duodenum (1) and the bend in the colon (2) mark the two points at which in the primitive schema ([Fig. 111], I.) the umbilical loop begins and terminates. The proximal of these two points (1) corresponds to the termination of the duodenum, which segment extends from here cephalad to the pyloric extremity of the stomach. The distal point (2) is placed on the colon where the returning limb of the loop resumes the original median vertical course of the large intestine. These two points mark the neck of the loop, which we can describe as the duodeno-colic neck or isthmus.