2 72 Vertebrate Embryology 



earlier stages in the development of the chick, 

 that the heart at first has no special cavity of 

 its own, but lies freely in the general body- 

 cavity (Figs. 55 and 57). The origin of the 

 pericardial cavity is, briefly, as follows : in the 

 plane of the Cuvierian veins, where they cross 

 the body-cavity from the somatopleure to the 

 sinus venosus, is developed a horizontal mem- 

 brane or septum. This septum evidently di- 

 vides the body-cavity into an upper and a 

 lower chamber, in the latter of which lies the 

 heart. These two chambers of the body- 

 cavity are at first in communication with each 

 other both in front and behind the horizontal 

 septum ; but the anterior edge of the septum 

 now grows forwards and upwards until it 

 meets and fuses with the ventral wall of the 

 fore-gut, and the posterior edge grows back- 

 wards and downwards until it meets the ven- 

 tral wall of the body-cavity ; the space in 

 which the heart lies is thus separated from the 

 rest of the body-cavity, and becomes the per- 

 icardial cavity. 



As the lungs are formed, as outgrowths 

 from the anterior part of the digestive tract, 

 they lie in the dorsal part of the body-cavity, 

 above the horizontal septum, and on each side 



