Ventrad and to the right of this line of adhesion we would trace that portion of the primitive membrane which now passes to the coils of the small intestine as the true mesentery, having an apparent origin in the background of the abdomen to the dotted line of adhesion. In the same manner the peritoneal layers passing to the left to reach the cæcum and beginning of the colon would appear as a free mesocolon with the same line of apparent origin from the background of the abdomen. (cf. p. 80.)
These considerations should be followed out in the dissection of the cat in order to become familiar with the principle of _secondary lines of origin_ for peritoneal layers. As we will see later this factor is of importance in correctly estimating the value of the human adult conditions.
3. A brief consideration of the mechanical conditions and comparison with the earlier stages will show why the peritoneal layers which occupy the bight of the fully developed umbilical loop are especially prone to develop secondary lines and areas of adhesion to other serous surfaces. If we compare the dorsal mesentery in its primitive condition, before the straight intestinal tube has become differentiated into the subsequent segments, and before the umbilical loop has been formed (Fig. 142), with the later stages represented by the intestines of the cat as now arranged (Figs. 143 and 144), it will be seen that the vertical line of attachment to the ventral surface of the vertebral column, between the points _a_ and _b_ corresponds in the advanced stages to the interval _ab_ separating the two points of the duodeno-colic isthmus; also that the entire mesenteric peritoneal surface beyond the isthmus is the result of drawing out and lengthening the intestinal tract. Consequently folding or overlapping of this extensive membrane affords opportunities for adhesions between its own serous surfaces or between it and the remaining visceral and parietal peritoneum of the abdomen.
[Illustration: FIGS. 142-144.--Schematic representation of three stages in the development of the mesentery of the umbilical intestinal loop.]
[Illustration: FIG. 142.--Early stage before differentiation of intestinal canal.]