628 THE PERICARDIAL CAVITY. 



Anteriorly all communication is thus early shut off between the 

 body cavity and the pericardial cavity, but the two cavities still 

 open freely into each other behind. 



The front part of the body cavity, lying dorsal to the peri- 

 cardial cavity, becomes gradually narrowed, and is wholly 

 obliterated long before the close of embryonic life, so that in 

 adult Elasmobranch Fishes there is no section of the body cavity 

 dorsal to the pericardial cavity. The septum dividing the body 

 cavity from the pericardial cavity is prolonged backwards, till it 

 meets the ventral wall of the body at the point where the liver 

 is attached by its ventral mesentery (falciform ligament). In 

 this way the pericardial cavity becomes completely shut off from 

 the body cavity, except, it would seem, for the narrow communi- 

 cations found in the adult. The origin of these communications 

 has not however been satisfactorily worked out. 



The septum between the pericardial cavity and the body 

 cavity is attached on its dorsal aspect to the liver. It is at first 

 nearly horizontal, but gradually assumes a more vertical position, 

 and then, owing to the obliteration of the primitive anterior 

 part of the body cavity, appears to mark the front boundary of 

 the body cavity. The above description of the mode of forma- 

 tion of the pericardial cavity, and the explanation of its relations 

 to the body cavity, probably holds true for Fishes generally. 



In the higher types the earlier changes are precisely the 

 same as those in Elasmobranch Fishes. The heart is at first 

 placed within the body cavity attached to the ventral wall of 

 the gut by a mesocardium (fig. 353 A). A horizontal septum is 

 then formed, in which the ductus Cuvieri are placed, dividing 

 the body cavity for a short distance into a dorsal (/./) and 

 ventral (p.c) section (fig. 353 B). In Birds and Mammals, and 

 probably also in Reptilia, the ventral and dorsal parts of the 

 body cavity are at first in free communication both in front of 

 and behind this septum. This is shewn for the Chick in 

 fig- 353 A an d B, which are sections of the same chick, A being 

 a little in front of B. The septum is soon continued forwards 

 so as completely to separate the ventral pericardial and the 

 dorsal body cavity in front, the pericardial cavity extending at 

 this period considerably further forwards than the body cavity. 



Since the horizontal septum, by its mode of origin, is 



