THE BODY CAVITY. 



517 



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^ 



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 pericardial 

 cavity become precisely those found in the embryos of Elasmo- 

 branchii. The later changes are however very different. Whereas 

 in Fishes the right and left sections of the body cavity dorsal to the 

 pericardial cavity soon atrophy, in the higher types, in correlation 

 with the relatively backward situation of the heart, they rapidly 

 become larger, and receive the lungs which soon sprout out from the 

 throat. 



The diverticula which form the lungs grow out into the splanchnic 

 mesoblast, in front of the body cavity ; but as they grow, they extend 

 into the two anterior compartments of the body cavity, each attached 



-de 



Fig. 353. Tbansvekse sections through a Chick embryo with twenty-one 

 mesoblastic somites to shew the formation of the pericardial cavity, a. being 

 the anterior section. 



pp. body cavity ; pc. pericardial cavity ; al. alimentary cavity ; au. auricle ; v. ven- 

 tricle ; sv. sinus venosus; dc. ductus Cuvieri ; ao. aorta; mp. muscle-plate; mc. 

 medullary cord. 



1 KoUiker'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. 



