EARLY DIVISION OF SPLANCHNOCOELIC COELOM 863 



(2) a larger caudal compartment, the peritoneal cavity, around the diges- 

 tive viscera and urogenital structures. 



This primary division of the early coelomic cavity is accomplished by the 

 formation of: 



1 ) The lateral mesocardia, and 



2) the primary (primitive) septum transversum. 



The two lateral mesocardia are formed previous to the development of the 

 primitive septum transversum. Eventually the lateral mesocardia fuse in part 

 to the dorsal edge of the transverse septum and become a part of it. The lateral 

 mesocardia thus, in reality, represent the initial stage in the division of the 

 general coelomic cavity. In consequence we shall consider the lateral meso- 

 cardia as important structures which enter into the formation of the primary 

 transverse division of the embryonic body, but they should not be confused 

 with the primitive septum transversum in a strict sense. 



a. Lateral Mesocardia 



The lateral mesocardia (fig. 362A-1, A-2) are formed as follows: 

 A lateral bulging or growth from the splanchnopleure at the caudal limits 

 of the developing sinus venosus extends dorso-laterad on each side to meet 

 a somewhat similar though smaller growth mediad of the somatopleural meso- 

 derm. These growths form a bridge on each side across the coelomic cavity, 

 extending dorso-laterad from the posterior lateral edges of the ventrally situ- 

 ated sinus venosus to the somatic wall. The area of union of this bridge on 

 either side with the lateral body wall is the lateral mesocardium. The lateral 

 mesocardia, in other words, represent the areas of juncture between the lateral 

 body walls and the lateral extensions of the sinus venosus. The common 

 cardinal veins or ducts of Cuvier join these right and left lateral extensions 

 or horns of the sinus venosus in the substance of the lateral mesocardia. An- 

 terior to the lateral mesocardia is the pericardial coelom, while posterior to 

 them is the peritoneal coelom. The two passageways dorsal to the lateral 

 mesocardia, on either side, are called the dorsal parietal recesses of His, 

 while those ventral to the lateral mesocardia and on either side of the ventral 

 mesentery and developing liver constitute the ventral parietal recesses of His 

 (fig. 362A). 



b. Formation of the Liver-Septum Transversum Complex 



1) Formation of Liver-Septum Complex through Modification of the Ven- 

 tral Mesentery by Liver Outgrowth. As the liver rudiment in the shark, 

 chick, pig, etc., grows ventrally and forward between the two splanchnopleural 

 layers of the ventral mesentery, it expands the ventral mesentery laterally as 

 the liver substance forms within the mesenchyme between the two splanchnic 

 layers. The expanding liver substance eventually reaches the ventral and lateral 



