In 12 there was an ascending mesocolon, but no corresponding fold on the left side. Hence from this series a mesocolon may be expected on the left side in 36 per cent., on the right side in 26 per cent. Both development and comparative anatomy would lead us to expect that the descending mesocolon would be found more frequently than the ascending. In the lower animals the descending mesocolon is always an extensive and conspicuous membrane. It is well developed in all monkeys and the anthropoidea, as the remains of the primary vertical fold of the dorsal mesentery, while the ascending mesocolon is a secondary production, acquired during the development of the bowel by rotation. In most of the lower monkeys the ascending mesocolon is also largely or entirely free. The descending mesocolon can always in these animals be reflected to the median line (cf. Fig. 155). [Illustration: FIG. 155.--Abdominal viscera of _Macacus cynomolgus_, Kra monkey, hardened _in situ_. (Columbia University Museum, No. 1801.)] The line of attachment in man of the descending mesocolon is usually along the lateral border of the left kidney and vertical, while the line of attachment of the ascending mesocolon is usually less vertical, crossing the caudal end of the right kidney obliquely from right to left and with an upward direction (Fig. 156). [Illustration: FIG. 156.--Schema of visceral and peritoneal relations of ventral surface of right kidney.] In like manner when both the ascending and descending mesocola are absent as free membranes the left or descending colon is adherent along the lateral border of the kidney to the abdominal parietes, while the ascending colon is fixed at the hepatic flexure a little obliquely across the ventral surface of the caudal end of the corresponding gland ascending toward the medial margin. Treves found in the cases of persistent ascending mesocolon in the adult that the membrane varied in breadth from 1" to 2" while the persistent fold on the left side varied between 2" and 3" in breadth. In the foetus, up to 5"-6" in length, the descending mesocolon is usually an extensive fold. Its attachment is vertical, but nearer to the median line than in the adult, usually along the medial border of the left kidney. It is at times found attached along this line in the adult. An ascending mesocolon is rare even in the foetus. The caecum and beginning of the ascending colon are completely invested by peritoneum, but above the parts so invested the colon is usually adherent along an oblique line to the ventral and medial aspect of the right kidney.