516 THE PERICARDIAL CAVITY. 



(pp), and (2) a ventral part the pericardia! cavity (pc). The septum 

 is at first of a very small longitudinal extent, so that both in front 

 and behind it (fig. 352 on the left side) the dorsal and ventral 

 sections of the body cavity are in free communication. The septum 

 soon however becomes prolonged, and ceasing to be quite horizontal, 

 is directed obliquely upwards and forwards till it meets the dorsal 

 wall of the body. 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 pericardial 

 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 communications 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 hori- 

 zontal, 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 formation 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 meso- 

 cardium (fig. 353 A). A horizontal septum is then formed, in which 

 the ductus Cuvieri are placed, dividiug the body cavity for a short 

 distance into a dorsal (pp) and ventral (pc} 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 and B, which are sections of the same chick, 

 A being a little in front of B. The septum is soon continued for- 

 wards so as completely to separate the ventral pericardial and the 

 dorsal body cavity in front, the pericardia! cavity extending at this 

 period considerably further forwards than the body cavity. 



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

 attached to the ventral side of the gut, the dorsal part of the primitive 

 body space is divided into two halves by a median vertical septum 

 formed of the gut and its mesentery (fig. 353 B). Posteriorly the 

 horizontal septum grows in a slightly ventral direction along the 

 under surface of the liver (fig. 354), till it meets the abdominal wall 



