EARLY DIVISION OF SPLANCHNOCOELIC COELOM 859 



and certain types of mesonephric tubules make contact with the splanchnocoelic 

 portion of the coelom. 



In all vertebrates (see figures 254, 332F-M) the formation of the primitive, 

 generalized coelomic cavity proper or generalized splanchnocoelic portion of 

 the coelom is formed by the fusion around the developing heart and gut struc- 

 tures of the two elongated splanchnocoels present in the hypomeric portions 

 of the mesodermal masses as described below. 



B. Early Divisions of the Primitive Splanchnocoelic Coelom 



1. Formation of Primitive Suspensory Structures 



The splanchnic walls of the early coelomic cavities (splanchnocoels) within 

 the two hypomeres become apposed around the structures, lying in the median 

 plane (fig. 254). In the region of the heart, this apposition gives rise to the 

 dorsal and ventral mesocardia and to the epimyocardium of the heart itself 

 (fig. 254A, B) and, in the region of the stomach and intestine, it produces 

 the dorsal and ventral mesenteries of the gut tube and various ligaments, 

 connecting one organ with another. The mesenchyme which arises from the 

 two splanchnic layers also gives origin to the muscles and connective tissues 

 of the gut and its evaginated structures (fig. 311 A, B). The ventral meso- 

 cardium disappears in all vertebrates (Chap. 17). The dorsal mesocardium 

 may persist for a while but eventually disappears entirely or almost entirely 

 (Chap. 17). The dorsal mesentery is present constantly in reptiles and mam- 

 mals but may be perforated and reduced in the intestinal area in other verte- 

 brate classes, so that little of the dorsal mesentery remains to suspend the 

 intestine in certain cases as, for example, in the shark. The dorsal mesentery 

 above the stomach, the mesogastrium, and also the ventral mesentery in the 

 immediate region between the stomach and liver and between the liver and 

 the ventral body wall persist in all vertebrates. As a rule, however, the ventral 

 mesentery disappears caudal to the liver with the exception of dipnoan and 

 anguilliform fishes and the ganoid fish, Lepisosteus. In these forms the ventral 

 mesentery tends to persist throughout the peritoneal cavity. It follows, there- 

 fore, that the two bilaterally developed, splanchnocoelic cavities tend to merge 

 into one cavity or generalized splanchnocoel with a partial retention in certain 

 areas of the splanchnic layers of the two hypomeres which act as suspensory 

 ligamentous structures for the viscera. 



2. Formation of the Primitive Transverse Division of the 



Body and the Primary Pericardial and Peritoneal 



Divisions of the Coelom 



The primitive splanchnocoelic coelom soon becomes divided into the peri- 

 cardial coelom, surrounding the heart, and the peritoneal or abdominal coelom, 

 surrounding the digestive viscera, by the formation of the lateral mesocardia 



