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 connect 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 



FIG. 139. Part of a transverse section 

 through the lateral mesocardia of a chick 

 with thirty-five pairs of somites (about 

 and Ventrallv Or ventro- seve nty-two hours). Af ter Lillie. a, Auricle; 



laterally, 

 11 



wall. 



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



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



"U7i^'i j-V, ron; CD, posterior cardinal vein; dC, due- 



While the septum tus ^ dwi> dorsal mesenter ' y . ; liver . 



tus 



SOOn becomes Z, lateral mesocardium; pc, pericardial 

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

 Complete between pen- groove; s, stomach; OT, sinus venosus; vm, 



cardial and body cavities, ventral mesenterv - 

 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 



