In the present study an attempt was made to think over anew the development of the peritoneal situs, guided by the comparison of the "root-area" (the connective tissue area free of peritoneum between the left and right transition line of the peritoneum parietale to the peritoneum viscerale) of the so-called "primitive mesentery" of an embryo (10 mm CRL) and of an older fetus (250 mm CRL). The following results were obtained: The so-called primitive sagittal mesentery cannot be regarded as the anlage of the adult mesentery, in the sense that the latter develops out of the former dy displacement of rotation. The original "root area" widens with the broadening of the back and can still be demonstrated as an homogeneous "root area" of the "intestinal bulge", after the typical adult situs has developed. There is a connection between the development of the mesenteries and the formation of the folds of the embryonic serosa. Their position is determined through the recesses of the intestinal bulge. They exist because of the foldings and are therefore secondary characteristics. Under these aspects it is possible to consider the development of the situs as a uniform folding process of the embryonic serosa, without references to as yet unproved secondary adhesions of the peritoneal layers.