DEVELOPMENTAL CHANGES IN THE EARLY EMBRYO 153 



layer of mesoderm, corresponding to the body somatic layer, will 

 become the pericardial membrane. The inner layer of mesoderm, 

 corresponding to the body splanchnic layer, will become the myo- 

 cardium or heart muscle. As in the body region, the mesoderm from 

 the two sides grows together ventrally to fuse below the foregut. 

 Both the coelom and the continuous and related pericardial cavity 

 arise as bilateral cavities only to fuse and form single cavities 

 around particular organs. 



The endocardium or lining of the heart arises from scattered 

 cells of mesodermal origin found beneath the pharynx. These cells 

 become organized into a sheet of epithelium as they are enclosed 

 by the bilateral folds of myocardium as they come together. 



