THE BODY CAVITY. 



629 



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 of the body at the 

 insertion of the falciform ligament, and thus completely shuts 

 off the pericardial cavity from the body cavity. The horizontal 

 septum forms, as is obvious from the above description, the 

 dorsal wall of the pericardial cavity 1 . 



A. 



B. 



FIG. 353. TRANSVERSE SECTIONS THROUGH A CHICK EMBRYO WITH TWENTY- 

 ONE MESOBLASTIC SOMITES TO SHEW THE FORMATION OF THE PERICARDIAL 

 CAVITY, A. BEING THE ANTERIOR SECTION. 



p.p. body cavity ; p.c. pericardial cavity ; a!, alimentary cavity ; an. auricle ; v. ven- 

 tricle; s.v. sinus venosus; d.c. ductus Cuvieri ; ao. aorta; tiif. muscle-plate; inc. 

 medullary cord. 



With the complete separation of the pericardial cavity from 

 the body cavity, the first period in the development of these 

 parts is completed, and the relations of the body cavity to the 



1 Kblliker's account of this septum, which he calls the mesocardium laterale (No. 

 298, p. 295), would seem to imply that in Mammals it is completed posteriorly even 

 before the formation of the liver. I doubt whether this takes place quite so early as 

 he implies, but have not yet determined its exact period by my own observations. 



