110 THE BODY-CAVITY. 



form a single cavity in which hangs the small intestine (PL x. 

 fig. 8) suspended by a very short mesentery. 



The most important change which takes place in the body- 

 cavity during this period is the formation of a septum which 

 separates off a pericardial cavity from the true body-cavity. 



Immediately in front of the liver the splanchnic and' 

 somatic w^alls of the body come into very close contact, and I 

 believe unite over the greater part of their extent. The septum 

 so formed divides the original body-cavity into an anterior 

 section or pericardial cavity, and a posterior section or true body- 

 cavity. There is left, however, on each side dorsalJy a rather 

 narrow passage which serves to unite the pericardial cavity in 

 front with the true body-cavity behind. 



In PL X. fig. 8 a, there is seen on one side a section through 

 this passage, while on the other side the passage is seen to be 

 connected with the pericardial cavity. 



It is not possible from transverse sections to determine for 

 certain whether the septum spoken of is complete. An exami- 

 nation of longitudinal horizontal sections from an embryo 

 belonfcino^ to the close of the sta^e K has however satisfied 

 me that this septum, by that stage at any rate, is fully formed. 



The two lateral passages spoken of above probably unite 

 in the adult to form the passage connecting the pericardial 

 with the peritoneal cavity, which, though provided with but a 

 single orifice into the pericardial cavity, divides into two limbs 

 before opening into the peritoneal cavity. 



The body-cavity undergoes no further changes of importance 

 till the close of the perio'i 



Bomatopleure and Splanclinopleure. Both the somatic and 

 splanchnic walls of the body-cavity during stage I exhibit a 

 simple uniform character throughout their whole extent. They 

 are formed of columnar cells where they line the dorsal part 

 of the body-cavity, but ventrally of more rounded and irregular 

 cells (PI. X. fig. 5). 



In them may occasionally be seen aggregations of very 

 peculiar and large cells with numerous highly refracting 

 spherules ; the cells forming these are not unlike the primitive 

 ova to be described subsequently, but are probably large cells 

 derived from the yolk. 



