THE CCELOM. 



lies dorsal to the pericardium, and between the fore -gut medially and the body 



laterally, is still a pleuro-pericardial canal. 



The Separation of the Pericardial, Pleural, and Peritoneal Parts of the 



Coelom. In the lateral wall of each 

 pleuro-pericardial canal, near its 

 cephalic end, lies the duct of Cuvier, 

 passing towards the heart ; and a 

 lung bud containing a primitive 

 bronchial tube grows, from the medial 

 wall, into the cavity of each pleuro- 



Spinal medulla 



-Li\ Alimentary canal 



Oesophagus 

 /Lung bud 



Opening into pericardium 

 Duct of Cuvier 



Pleuro-pericardial canal 

 .. Lung bud 



Commencing lateral 

 i , part of diaphragm 



Septum transversum 



- -Peritoneum 



^ ^ 



FIG. 91. SCHEMA OF LATER STAGE OF DIFFERENTIATION 



OF CCELOM. A, from above. B, transverse section cut FIG. 92. SCHEMA OF A TRANSVERSE SECTION 

 level of lung bud in A. AT THE LEVEL OF THE LUNG BUD IN FIG. 91. 



pericardial canal (Fig. 91). As the lung buds grow the cavities of the pleuro- 

 pericardial canals increase in size, and each passes ventrally, round the side of 



the pericardium towards the ven- 

 tral wall of the body, until it is 

 separated from its fellow of the op- 

 posite side only by a median meso- 

 derm-filled interval, which becomes 

 the anterior mediastinum and the 

 anterior part of the superior media- 

 pieurai cavity stinum (Fig. 94). At the same time 



closed aperture between the cavitv of each pleuro-pericardial 



pleura and pericardium . -, ,-, i T i 



Duct of Cuvier canal, and the growing lung bud 



in its interior, grow towards the 

 cephalic end of the embryo (Fig. 



B- 



Lung 

 Bronchus 



Lateral part of 



diaphragm converging 



towards dorsal 



mesentery 



Septum transversum 



Peritoneum 



Spinal medulla 

 (Esophagus 



FIG. 93. SCHEMA OF STILL LATER STAGE OF CCELOM 

 DIFFERENTIATION. The pleurae are separated from the 

 pericardia, but still communicate with the peritoneum. 



FIG. 94. SCHEMA OF TRANSVERSE 

 SECTION OP EMBRYO AT LEVEL OF 

 LINE B, Fig. 93, showing ventral ex- 

 tension of the pleurae. 



93). As it passes cephalwards the growing lung lies to the lateral side of the 

 duct of Cuvier, which is thus forced against the cephalic end of the pleuro- 

 perieardiaj[ canaj, compressing it towards the median plane, against the sides 



