208 



THE DEVELOPMENT OF THE CHICK 



instead of in front of them as before. Therefore in cross-sections, in 

 front of the lateral mesocardia, the pleural grooves appear as dorsal 

 projections of the parietal (later pericardial) cavity, separated from 

 one another in the middle line by the oesophagus (Fig. 118). 



The relations of the three divisions of the embryonic body- 

 cavity thus established may be described as follows: the parietal 

 cavity contains the heart, and is therefore the prospective peri- 



CA 



yof- 



m. 



1^ 



^■t.dors- 



Fig. 118. — Transverse section of an embryo of 35 s, imme- 

 diately in front of the lateral mesocardia. 

 Ao., Aorta. Atr., Atrium. B. a., Bulbiis arteriosus. D.C. 

 r , and'l., Ri^ht and left ducts of Cuvier. Lg., Lung, m's'c. 

 dors., Dorsal' mesocardium. m's't. dors., Dorsal mesentery. 

 P. C, Pericardial cavity, pi. gr., Pleural groove. Rec. pul. 

 ent., Recessus pulmo-entericus. S. V., Sinus venosus. 



cardial cavity. It is not, however, a closed cavity, but communi- 

 cates in front of the lateral mesocardia with the pleural grooves 

 (Fig. 118), and by way of the latter above the lateral mesocardia 

 with the peritoneal cavity (Figs. 119 and 120); a second communi- 

 cation of the parietal cavity with the peritoneal cavity is beneath 

 the lateral mesocardia around the sides of the anterior intestinal 

 portal, now being converted into the septum transversum (cf. 



