Let's digress for a few minutes, and look at a sequence of highly simplified diagrams.
The foregut starts as a straight tube. As it develops, it rotates on its long axis, lengthens in a double curve, and expands to become the stomach, and the first part of the duodenum.
The foregut is different from the rest of the G-I tract. The hindgut and midgut are attached to the body wall by a double fold of peritoneum only along the back. The foregut is attached also at the front. Its two attachments are the dorsal mesogastrium behind, and the ventral mesogastrium in front. As the foregut rotates, the dorsal and ventral mesogastrium rotate with it.
The line of attachment of the ventral mesogastrium swings round to the right as the foregut develops. It ends up running along the lesser curve of the stomach, and the top of the proximal duodenum.
On the back, the attachment of the dorsal mesogastrium swings round to the left. It ends up running along the greater curve of the stomach, and the underside of the proximal duodenum. While the foregut is developing, there are important changes in the ventral and dorsal mesogastrium.
The liver develops in the ventral mesogastrium, the spleen develops in the dorsal mesogastrium. The liver grows rapidly, pressing against the body wall, and obliterating these layers of peritoneum. These changes produce this almost separate pocket behind the stomach, the lesser sac.
This part of the ventral mesogastrium is the lesser omentum. This part of the dorsal mesogastrium will become the greater omentum. We'll follow these changes from the start in a more three-dimensional way. To do that, we'll go to a view from below.
Here we're looking up into the upper part of the abdominal cavity This is the diaphragm. Here's the foregut, starting to develop, here's the liver developing in the ventral mesogastrium, and the spleen developing in the dorsal mesogastrium.
Here's the space that will be the lesser sac. This is the lesser omentum, this part of the dorsal mesogastrium will grow downwards to become the greater omentum. We'll move to a slightly lower vantage point so we can add the duodenum to the picture. The foregut ends here: so does the lesser omentum. This is the lower free border of the lesser omentum.
Below it the duodenum becomes stuck againt the liver, leaving this opening, the epiploic foramen, that leads into the lesser sac. The dorsal mesogastrium is shown as though it had a free lower border along here, but in reality this fold is continued all the way round to here, creating a sac that has only one opening, here.
To see how the greater omentum develops we'll first add the transverse colon to the picture. The dorsal mesogastium hangs down in front of the transverse colon. To follow its growth we'll look at a sagittal section made in this line.
Here's the lesser omentum, between the liver and the stomach. Here's the greater omentum, hanging down in a double fold. Below it is the transverse colon, suspended by the transverse mesocolon. This is the pancreas. The greater omentum grows downward in front of the transverse colon.
The greater omentum and the transverse mesocolon come together, and the duplicated layers are absorbed, so that we're left with the greater omentum stuck to the transverse colon, and hanging down below it. The lesser sac lies behind the lesser omentum, the stomach, and this part of the greater omentum, the gastro-colic ligament.
Now that we've seen, in a very diagrammatic way, how these structures developed, you may find it helpful either now or later to take another look at the previous five minutes of the tape.