THE LATER DEVELOPMENT OF THE CHICK 351 



two cavities, but they still remain connected above and below 

 this partition. The pericardial cavity soon becomes restricted 

 to the median region of the body and the general body cavity 

 then pushes forward along the sides of the pericardial cavity. 

 These antero-lateral extensions of the body cavity are the rudi- 

 ments of the pleural cavity; they soon extend inward toward 

 the median line, dorsally 

 to the pericardial cavity 

 with which, however, they 

 still connecj above the 

 lateral mesocardia. The 

 pericardial cavity still 

 connects with the general 

 body cavity beneath the 

 lateral mesocardia (Fig. 

 139). 



The complete closure of 

 the pericardial cavity is 

 begun during the fourth 

 day, by the formation of 

 the septum transversum. 

 This partition is estab- 

 lished by the formation of 

 tissue connecting the 

 lateral mesocardia dorsally with thirty-five pairs of somites (about 



and Ventrallv Or Ventro- seven ty-t wo hours). Af ter Lillie. a, Auricle; 



. acm, accessory mesentery; am, amnion; ao, 



laterally, With the body dorsal aorta; ba, bulbus arteriosus; ch, cho- 



11 nn, 'i "U rion; cv, posterior cardinal vein; dC, due- 



wall. While the septum tus Cuvieri; dm> dorsal mesentery . t> i iver; 



lm, lateral mesocardhwn ; pc, pericardial 

 cavity; pe, pulmo-enteric recess; pg, pleural 

 groove; s, stomach; sv, sinus venosus; vm, 



cardial and body cavities, ventral mesenter y- 

 it remains for a time incomplete between pleural and body 

 cavities. As the lungs begin to expand they push out into 

 the pleural cavities, the walls of which supply the greater 

 part of their mesodermal tissue. Finally the lungs extend 

 posteriorly as well as laterally and as they reach the region of 

 the septum transversum this gradually becomes completed 



FIG. 139. Part of a transverse section 

 through the lateral mesocardia of a chick 



transversum soon becomes 

 complete between peri- 



