I 8 INTRODUCTION 



and liver. Then there are also gastrosplenic, duodeno-hepatic omenta, the 

 positions of which are readily understood. The mammals have a great omen- 

 tum, a double fold of mesogaster and mesocolon which connects stomach and 

 colon and forms a large sac, the bursa omentalis, the cavity of which is connected 

 with the rest of the body cavity by a small opening (foramen epiploicimi or 

 foramen of Winslow) near the hinder end of the liver. 



Frequently the reproductive glands (gonads) project so far into the body 

 cavity that the serosa meets behind them, forming supports for them similar to 

 mesenteries. These are called mesorchia for the testes, mesovaria for the 

 ovaries. 



In the early embryo the metacoeles extend as well-developed 

 cavities from just behind the head back to the anus. Each is soon 

 divided by a cross partition, the septum transversum, into a smaller 

 anterior cavity, the pericardium, which surrounds the heart, and a 

 larger splanchnocoele or body cavity which includes the rest of the 

 viscera. The method of the formation of the septum transversum 

 is described in connexion with the heart (p. 294), but here it may be 

 stated that, just in front of the liver a pair of blood-vessels, the 

 Cuvierian ducts, enter the heart from the sides. These arise in the 

 somatopleure and as they increase in diameter, they project into the 

 ccelom, carrying the somatic serosa before them. The fold thus 

 produced is the septum transversum which is attached to the front 

 margin of the liver. In many lower vertebrates {e.g., elasmobranchs) 

 this septum is not complete, failing to reach the other (splanchnic) 

 wall, thus leaving one or more openings — ^pericardio-peritoneal 

 canals — connecting the pericardium with the body cavity. In the 

 higher vertebrates the closure is complete. 



In the formation of the pericardial cavity double serosal mem- 

 branes (mesocardia) are formed which are similar to the mesenteries 

 in origin and structure (see fig. 312). These are temporary and in all 

 vertebrates disappear in the earlier stages of development. 



In all the lung-bearing vertebrates except the mammals, the lungs 

 are suspended in the general body cavity (splanchnocoele) , but in the 

 latter group a second partition, the diaphragm, cuts off a pair of 

 pleural cavities from the rest of the splanchnocoele which is now 

 called the peritoneal cavity. Traces of structures similar to the 

 diaphragm, as indicated by nerve supply, etc., occur in vertebrates 

 as low as the amphibia, but their exact homology is uncertain. The 

 development of the mammalian diaphragm is a complicated process 

 and is stated here only in outline. It involves a part of the septum 

 transversum, but is largely a new formation. At first a part of the 



