200 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



the hinder end of the heart where there exists on each side a broad 

 bridge by which the duct of Cuvier passes from the somatopleure tff 

 the sinus venosus. This bridge becomes extended headwards and 

 dorsally on each side of the oesophagus until it meets the dorsal 

 wall of the splanchnocoele thus forming with the oesophagus a 

 floor separating the anterior portion of the splanchnocoele into two 

 cavities, one dorsal and one ventral, each opening posteriorly into 

 the main splanchnocoele. Of these two cavities the dorsal becomes 

 completely obliterated by fusion of its splanchnic (oesophageal) and 

 somatic walls from before tailwards. The ventral one on the 

 other hand roofed in by the oesophagus persists as the pericardiac 

 cavity. 



The communication of this posteriorly with the main splanchno- 

 coele is obstructed in the middle by the flattened headward surface 

 of the liver which is embedded in the distended ventral mesentery, 

 while laterally the communication is for a time open. As develop- 

 ment goes on however the opening on each side becomes obliterated 

 by an ingrowth from the somatopleure which spreads downwards 

 from the bridge of tissue containing the duct of Cuvier and the 

 free edge of which meets and fuses with the mesoderm covering the 

 headward surface of the liver. The pericardiac cavity comes in this 

 way to be bounded posteriorly by a complete wall of tissue a large 

 part of which consists simply of the mesodermal sheath of the liver. 

 As the body of the embryo increases in diameter this wall of tissue 

 keeps pace with it as does also the liver. The latter organ however 

 in subsequent growth of its anterior or headward surface does not 

 keep growing in continuity with the substance of the septum but 

 becomes separated from it by a deep cleft, the region of continuity 

 between liver and septum becoming thus restricted to a small 

 area dorsal and close to the mesial plane. Similarly the region 

 of continuity between the headward face of the septum and the 

 wall of the sinus venosus which is at first of relatively considerable 

 dorsiventral extent becomes reduced to a narrow bridge of tissue. 



In the Elasmobranchs the isolation of pericardiac cavity from 

 the main splanchnocoele is only temporary. A median pocket-like 

 extension of the pericardiac cavity spreads tailwards immediately 

 dorsal to the sinus venosus in the substance of the mesodermal 

 sheath covering the ventral surface of the oesophagus. This develops 

 on each side a communication with the main cavity of the splanchno- 

 coele which persists throughout life as a crescentic slit on the ventral 

 surface of the oesophagus (Hochstetter, 1900). This secondary com- 

 munication between pericardiac coelome and splanchnocoele is 

 known as the pericardioperitoneal canal. 



In Myxinoids, throughout life, and in Petromyznn, during the 

 larval period, the rudiment of the wall separating pericardiac from 

 splanchnocoelic cavity remains in the form of a simple bridge 

 enclosing the duct of Cuvier so that the two cavities are in wide 

 communication with one another. 



