344 THE DEVEL0P:\IEXT OF THE CHICK 



At about ninety-six hours, the pUca 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 

 caval fold; the more median division is the coeliac fold including 

 the coeliac arter}^ Between them is a subdivision of the recesses 

 known as the cavo-coeliac recess, which corresponds to the atrium 

 burs£e 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 coeliac 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 



