It will be readily appreciated that a comparatively flat organ like the stomach, will, as long as it occupies a sagittal position, with right and left surfaces, help to divide the upper part of the abdominal cavity to a certain extent into a right and left half, even if the peritoneal connections of the organ are left out of consideration. As soon, however, as the above-described changes in position take place and the surfaces of the stomach are directed ventrad and dorsad, the relative arrangement and extent of this right and left abdominal space becomes altered by the different disposition of the septum, _i. e._, the stomach. The original right side of the organ is now directed dorsad, and the rotation of the organ has created a space between this dorsal or "posterior" surface of the stomach and the background of the abdominal cavity, which is the inception of the "lesser peritoneal cavity" or retrogastric space. We will find that this space becomes well defined and circumscribed by the peritoneal connections of the stomach, but we will realize, even at this stage, that the _dorsal_ surface of the stomach will form a part of the general _ventral_ wall of the lesser peritoneal space. On the other hand, the partial division of the abdomen into a right and left half, effected by the stomach in its primitive sagittal position, disappears after rotation of the organ. We now pass uninterruptedly from left to right across the ventral (original _left_) surface of the stomach. B. Changes in the Direction and Extent of the Dorsal Mesogastrium. The effects of the altered position of the stomach on the disposition of the abdominal space have just been considered in relation to the organ itself, without reference to its natural connections with the parietes and with adjacent viscera. Their true significance and their influence on the adult anatomical arrangement of the abdomen is, however, only appreciated when the changes in the arrangement of the peritoneal membrane which they involve, are taken into account. The dorsal mesogastrium changes more than any other portion of the peritoneum in the course of development. It not only becomes displaced and altered in direction by the rotation of the stomach, but in addition it grows so extensively that it finally hangs down like an apron over the entire mass of small intestines, forming the great omentum. If we begin with the primitive disposition of the sagittal stomach and dorsal mesogastrium shown in Fig. 171 it will be observed that both structures together actually divide the dorsal portion of the abdominal cavity into symmetrical right and left halves (Fig. 172). [Illustration: FIG. 171.--Schematic representation of dorsal mesogastrium before rotation of stomach.] [Illustration: FIG. 172.--Semi-diagrammatic representation of mesogastrium in human embryo of the sixth week. (Kollmann.)]