344 THE DEVELOPMENT OF THE CHICK 
At about ninety-six hours, the plica mesogastrica divides to 
form two longitudinal folds, in the lateral one of which the vena 
cava inferior develops (cf. Fig. 193 B); it is hence known as the 
‘aval fold; the more median division is the coeliac fold including 
the coeliac artery. Between them is a subdivision of the recesses 
known as the cavo-cceliac recess, which corresponds to the atrium 
bursee omentalis of mammals. The fusion of the right lateral 
border of the liver continues along the course of the caval fold, 
and the vena cava inferior is soon completely enveloped in liver 
tissue. Behind the point where the vena cava inferior enters 
the liver, the latter fuses with the ventral edge of the right meso- 
nephros, thus progressively diminishing the opening of the collec- 
tive recesses into the peritoneal cavity. At about the one hun- 
dred and sixtieth hour, the fusion reaches the portal vein, and the 
recesses are thus completely shut off from the peritoneal cavity. 
Thus a lesser peritoneal cavity is completely separated on the 
right side of the body from the main cavity; and from the former 
both lesser and greater omental spaces develop on the right and 
left sides respectively of the coeliac fold. (Bursa omenti minoris 
and bursa omenti majoris of the bursa omentalis dextra.) 
The communication of the lesser and greater omental spaces 
in front of the cceliac fold is closed by fusion of the latter with 
the right side of the proventriculus at about the one hundred 
and sixtieth hour, though it remains open throughout life in some 
birds. The two omental spaces are also elongated in a posterior 
direction by the caudal prolongation of the right lobe of the liver 
and of the gizzard respectively (Fig. 195). The lateral wall of 
the omentum minus is attached to the lateral dorsal border of 
the right lobe of the liver as already described, and it is therefore 
carried back by the elongation of this lobe; but as the vena cava 
inferior is inserted about the middle of this wall and cannot be 
drawn back, it results that there is a deep median indentation 
of the lateral wall of the omentum minus, at the bottom of which 
lies the vena cava inferior. 
The condition of both right and left omental spaces at 154 
hours is shown in Figures 195 and 196. Subsequently, about the 
eleventh day, the mesogastrium behind the spleen becomes per- 
forated, and the greater omental space thus opens secondarily 
into the left side of the body-cavity. A true omental fold exists 
only for a short time in the development of the chick, and is 
