328 DEVELOPMENT OF ELASMOBRANCH FISHES. 



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 

 walls 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 dorsally a rather narrow 

 passage which serves to unite the pericardial cavity in front with 

 the true body-cavity behind. 



In PI. n, 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 be- 

 longing to the close of the stage 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 period. 



Somatopleure and Splanchnopleure. 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. 11, 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. 



It is during the stage intermediate between I and K that the 

 first changes become visible which indicate a distinction between 



